Liquid crystal display device

ABSTRACT

A liquid crystal (LC) display device includes a LC panel and a driving circuit. The LC panel exhibits, in its voltage-transmittance characteristics, an extreme transmittance at a voltage equal to or lower than a lowest gray-level voltage. The driving circuit supplies to the LC panel a predetermined driving voltage overshooting a gray-level voltage corresponding to an input image signal of a current vertical period, according to a combination of an input image signal of an immediately preceding vertical period and the input image signal of the current vertical period.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to a liquid crystaldisplay device (LCD). More particularly, the present invention relatesto an LCD preferably used for moving picture display.

[0003] 2. Description of the Background Art

[0004] The LCDs are used for, e.g., personal computers, word processors,amusement equipments, television sets, and the like. Improvement inresponse characteristics of the LCDs has been studied for high-qualitymoving picture display.

[0005] Japanese Laid-Open Publication No. 4-288589 discloses an LCDhaving an increased response speed for intermediate-gray-scale displayin order to reduce a residual image. In this LCD, an input image signalhaving its high-band components pre-enhanced is supplied to a liquidcrystal display section so that the rise and fall speeds of the responseare increased. Note that the “response speed” in the LCDs (liquidcrystal panels) corresponds to an inverse number of the time requiredfor the liquid crystal layer to reach an alignment state correspondingto the applied voltage (i.e., response time). The structure of a drivingcircuit of this LCD will be described with reference to FIG. 21.

[0006] The driving circuit of the aforementioned LCD includes an imagestorage circuit 61 for retaining at least one field image of an inputimage signal S(t), and a time-axis filter circuit 63 for detecting avariation in level of each picture element in the time-axis direction,based on the image signal retained in the storage circuit 61 and theinput image signal S(t), and filtering the input image signal S(t) forhigh-band enhancement in the time-axis direction. The input image signalS(t) is a video signal decomposed into R (Red), G (Green) and B (Blue)signals. Since the R, G and B signals are subjected to the sameprocessing, only one channel is shown herein.

[0007] The input image signal S(t) is retained in the image storagecircuit 61 for storing an image signal of at least one field. Adifference circuit 62 calculates the difference between respectivepicture-element signals of the input image signal S(t) and the imagesignal stored in the image storage circuit 61. Thus, the differencecircuit 62 serves as a level variation detection circuit for detecting avariation in signal level during a single field. A difference signalSd(t) in the time-axis direction from the difference circuit 62 is inputtogether with the input image signal S(t) into the time-axis filtercircuit 63.

[0008] The time-axis filter circuit 63 is formed from a weightingcircuit 66 for weighting the difference signal Sd(t) with a weightcoefficient a corresponding to the response speed, and an adder 67 foradding the weighted difference signal and the input image signal S(t)together. The time-axis filter circuit 63 is an adaptive filter circuitwhose filter characteristics can be varied according to the output ofthe level variation detection circuit and the input level of eachpicture element of the input image signal. This time-axis filter circuit63 enhances the input image signal S(t) in its high band in thetime-axis direction.

[0009] The high-band enhanced signal thus obtained is converted into analternating current (AC) signal by a polarity inversion circuit 64, andthis AC signal is supplied to a liquid crystal display section 65. Theliquid crystal display section 65 is an active-matrix liquid crystaldisplay section including display electrodes (also referred to aspicture-element electrodes) at the respective intersections of aplurality of data signal lines and a plurality of scanning signal linescrossing the same.

[0010]FIG. 22 is a signal waveform chart illustrating how the responsecharacteristics are improved with this driving circuit. For simplicityof the description, it is herein assumed that the input image signalS(t) changes with a cycle period of one field, and the figure shows thecase where the signal level rapidly changes in two fields. In this case,as shown in the figure, a change in the input image signal S(t) in thetime-axis direction, i.e., the difference signal Sd(t), becomes positivefor one field in response to the input image signal S(t) changing topositive, and becomes negative for one field in response to the inputimage signal S(t) changing to negative.

[0011] Basically, high-band enhancement can be achieved by adding thedifference signal Sd(t) to the input image signal S(t). Actually, therelation between the respective degrees of change in the input imagesignal S(t) and in the transmittance depends on the response speed ofthe liquid crystal layer. Therefore, the weight coefficient a isdetermined so as to make correction within the range that does not causeany overshoot. As a result, a high-band enhanced high-band correctionsignal Sc(t) as shown in FIG. 22 is input to the liquid crystal displaysection, whereby optical response characteristics I(t) are improved asshown by the solid line over a conventional example shown by the dashedline.

[0012] In the case where the driving circuit as disclosed in theaforementioned publication is applied to a current LCD, responsecharacteristics at a rise (a change to the display state correspondingto an increase in voltage applied to the liquid crystal layer) can beimproved. However, the effect of improving the response characteristicsat a fall (a change to the display state corresponding to a decrease involtage applied to the liquid crystal layer) is relatively poor. In theLCD, a fall indicates a relaxation phenomenon that the liquid crystalmolecules are restored from the orientation state corresponding to afirst voltage toward that corresponding to a second voltage that islower than the first voltage. The time required for the liquid crystalmolecules to reach the orientation state corresponding to the secondvoltage (fall response time) mainly depends on the restoring forceacting between the liquid crystal molecules. Accordingly, in the casewhere the voltage applied to the liquid crystal layer reduces from thefirst voltage to the second voltage, the fall response speed (or fallresponse time) of the liquid crystal layer generally does not so muchdepend on the magnitude of the second voltage (the difference from thefirst voltage). Therefore, the effect of increasing the fall responsespeed is poor even if the input image signal S(t) is emphasized.

[0013] It is now assumed that the lowest gray-level voltage (the lowestvalue of the gray-level voltage) is set to the value corresponding tothe maximum transmittance in the LCD having such voltage-transmittance(V-T) characteristics as shown in FIG. 20 of the aforementioned JapaneseLaid-Open Publication No. 4-288589 (corresponding to the V-T curve of260-nm retardation in FIG. 5A of the present application). Particularlyin this case, the fall response speed cannot be increased even if anovershoot voltage (a voltage lower than the lowest gray-level voltage)is applied. The reason for this is as follows: the orientation state ofthe liquid crystal molecules is substantially the same within a voltageregion corresponding to the maximum transmittance (a flat region of theV-T curve). Therefore, the restoring force acting between the liquidcrystal molecules is substantially the same whatever voltage within thisregion is applied.

[0014] As described above, the terms “rise” and “fall” as used in thespecification correspond to a change in display state (or orientationstate of the liquid crystal layer) according to an “increase” and“decrease” in voltage applied to the liquid crystal layer, respectively.A “rise”, which is a change with an increase in applied voltage,corresponds to a “reduction in brightness” in the normally white mode(hereinafter, referred to as “NW mode”) and to an “increase inbrightness” in the normally black mode (hereinafter, referred to as “NBmode”). A “fall”, which is a change with a decrease in applied voltage,corresponds to an “increase in brightness” in the NW mode and to a“reduction in brightness” in the NB mode. In other words, a “fall” isassociated with the relaxation phenomenon of the orientation of theliquid crystal layer (liquid crystal molecules).

[0015] Moreover, the driving method disclosed in the aforementionedJapanese Laid-Open Publication No. 4-288589 has a problem that the inputimage signal S(t) capable of being subjected to effective high-bandenhancement is limited. More specifically, the high-band correctionsignal Sc(t) cannot exceed a high-band limit signal (which is hereindefined as a signal having the highest voltage among the input imagesignals s(t) that are input to the liquid crystal display section).Therefore, the input image signal can be subjected to high-bandenhancement if the high-band correction signal Sc(t)≦the high-band limitsignal. However, if the high-band correction signal Sc(t)>the high-bandlimit signal, a correction signal enough to cause a sufficient change intransmittance cannot be input to the liquid crystal display section.Accordingly, the response speed is increased at an intermediate graylevel, but the effect of improving the optical response characteristicsis reduced at a higher band level (as the voltage applied to the liquidcrystal display section is increased).

[0016] The present invention is made in view of the aforementionedproblems, and it is an object of the present invention to provide an LCDwith improved fall response characteristics. It is another object of thepresent invention to provide an LCD with improved responsecharacteristics at least at a high-band level.

SUMMARY OF THE INVENTION

[0017] A liquid crystal display device according to a first aspect ofthe present invention includes: a liquid crystal panel including aliquid crystal layer and an electrode for applying a voltage to theliquid crystal layer; and a driving circuit for supplying a drivingvoltage to the liquid crystal panel, wherein the liquid crystal panelexhibits, in its voltage-transmittance characteristics, an extremetransmittance at a voltage equal to or lower than a lowest gray-levelvoltage, and the driving circuit supplies to the liquid crystal panel apredetermined driving voltage overshooting a gray-level voltagecorresponding to an input image signal of a current vertical period,according to a combination of an input image signal of an immediatelypreceding vertical period and the input image signal of the currentvertical period. Thus, the object of the present invention, i.e.,improved fall response characteristics, is achieved.

[0018] Preferably, a difference in retardation of the liquid crystalpanel between a state where a voltage is not applied and a state where ahighest gray-level voltage is applied is 300 nm or more.

[0019] Preferably, the liquid crystal panel is a transmission-typeliquid crystal panel, and the extreme transmittance provides a maximumtransmittance.

[0020] A single vertical period of the input image signal may correspondto a single frame, at least two fields of the driving voltage maycorrespond to a single frame of the input image signal, and the drivingcircuit may supply, at least in a first field of the driving voltage, adriving voltage overshooting a gray-level voltage corresponding to aninput image signal of a current field to the liquid crystal panel.

[0021] Preferably, the liquid crystal layer is a homogeneous-orientationliquid crystal layer.

[0022] The liquid crystal panel may further include a phase compensator,three principal refractive indices na, nb and nc of an index ellipsoidof the phase compensator may have a relation of na=nb>nc, and the phasecompensator may be arranged so as to cancel at least a part ofretardation of the liquid crystal layer.

[0023] A liquid crystal display device according to a second aspect ofthe present invention includes: a liquid crystal panel including aplurality of picture-element capacitors arranged in a matrix, and thinfilm transistors respectively electrically connected to the plurality ofpicture-element capacitors; and a driving circuit for supplying adriving voltage to the liquid crystal panel, wherein the liquid crystaldisplay device updates display every vertical period by rendering theplurality of picture-element capacitors in a charged state correspondingto the input image signal, each of the plurality of picture-elementcapacitors includes a liquid crystal capacitor formed from acorresponding picture-element electrode, a counter electrode and aliquid crystal layer provided between the picture-element electrode andthe counter electrode, and a storage capacitor electrically connected inparallel with the liquid crystal capacitor, a capacitance ratio of thestorage capacitor to the liquid crystal capacitor being 1 or more, andthe picture-element capacitor retains 90% or more of a charging voltageover a single vertical period, when at least a highest gray-levelvoltage is applied. Thus, the object of the present invention, i.e.,improved response characteristics at least at a high-band level, isachieved.

[0024] Preferably, the driving circuit supplies to the liquid crystalpanel a predetermined driving voltage overshooting a gray-level voltagecorresponding to an input image signal of a current vertical period,according to a combination of an input image signal of an immediatelypreceding vertical period and the input image signal of the currentvertical period.

[0025] For the input image signal of every gray level, the drivingcircuit may supply to the liquid crystal panel the driving voltageovershooting the gray-level voltage corresponding to the input imagesignal of the current vertical period.

[0026] The liquid crystal layer of the liquid crystal panel may includea nematic liquid crystal material having a positive dielectricanisotropy, the liquid crystal layer included in each of the pluralityof picture-element capacitors may include first and second regionshaving different orientation directions, and the liquid crystal panelmay further include a pair of polarizers arranged so as to orthogonallycross each other with the liquid crystal layer interposed therebetween,and a phase compensator for compensating for a refractive indexanisotropy of the liquid crystal layer in a black display state.

[0027] Alternatively, the liquid crystal layer may be ahomogeneous-orientation liquid crystal layer.

[0028] Preferably, the liquid crystal panel further includes a phasecompensator, three principal refractive indices na, nb and nc of anindex ellipsoid of the phase compensator have a relation of na=nb>nc,and the phase compensator is arranged so as to cancel at least a part ofretardation of the liquid crystal layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a graph showing V-T curves of a liquid crystal panelthat includes a parallel-orientation liquid crystal layer including aliquid crystal material with a positive refractive index anisotropy(Δn=n//−n⊥>0).

[0030]FIG. 2A is a graph showing a voltage-retardation curve of a liquidcrystal panel having a retardation of 260 nm.

[0031]FIG. 2B is a graph showing a voltage-retardation curve of a liquidcrystal panel having a retardation of 300 nm.

[0032]FIG. 3 is a schematic diagram showing the relation between a V-Tcurve, dedicated overshoot-driving voltage Vos and gray-level voltage Vgin a liquid crystal panel included in an LCD according to an embodimentof the present invention.

[0033]FIG. 4 is a schematic diagram showing the structure of a drivingcircuit 10 included in the LCD according to the embodiment of thepresent invention.

[0034]FIG. 5A is a graph showing the respective V-T curves of the LCDaccording to the embodiment of the present invention (liquid crystalpanel with 320-nm retardation) and an LCD of a comparative example(liquid crystal panel with 260-nm retardation), and also showing theconditions of setting the lowest gray-level voltage.

[0035]FIG. 5B is a graph schematically showing a change in transmittancewith time in the LCD according to the embodiment of the presentinvention.

[0036]FIG. 5C is a graph showing the respective V-T curves of the LCDaccording to the embodiment of the present invention (liquid crystalpanel with 320-nm retardation) and an LCD of a comparative example(liquid crystal panel with 260-nm retardation), and also showing theconditions of setting the lowest gray-level voltage.

[0037]FIG. 5D is a graph schematically showing a change in transmittancewith time in the LCD according to the embodiment of the presentinvention.

[0038]FIG. 6 is a graph schematically showing a change in transmittancewith time in another LCD of the embodiment.

[0039]FIG. 7 is a diagram schematically showing a NW-modetransmission-type liquid crystal panel using a parallel-orientationliquid crystal layer, which is included in the LCD according to theembodiment of the present invention.

[0040]FIG. 8 is a diagram illustrating functions of a phase compensatorused in the embodiment.

[0041]FIG. 9 is a graph showing the effects of the thickness of thephase compensator on the V-T curve of the liquid crystal panel.

[0042]FIG. 10 is a diagram schematically showing an LCD 30 according tothe embodiment of the present invention.

[0043]FIG. 11 is a diagram illustrating response characteristics of theLCD 30 of the present embodiment, wherein an input image signal S, atransmittance, and a voltage that is output to the liquid crystal panelare shown together with a comparative example.

[0044]FIG. 12 is a schematic diagram showing a TFT-type LCD according toa second embodiment of the present invention.

[0045]FIG. 13 is a schematic diagram illustrating a step response in theTFT-type LCD.

[0046]FIG. 14 is a diagram schematically showing a change intransmittance with time when the gray level of an input image signal ischanged.

[0047]FIG. 15 is a graph showing a change in transmittance in NW modeLCDs having various Cs/Clc values in the case where the input imagesignals (gray-level voltages) of the previous and current fields aredifferent from each other.

[0048]FIG. 16 is a diagram showing a change in transmission with timeaccording to a change in gray-level voltage (input image signal).

[0049]FIG. 17 is a diagram schematically showing an NB modetransmission-type liquid crystal panel using a parallel-orientationliquid crystal layer, which is included in the LCD according to theembodiment of the present invention.

[0050]FIG. 18A is a diagram showing response characteristics of an LCDaccording to a third embodiment of the present invention.

[0051]FIG. 18B is a diagram showing a driving voltage of the LCDaccording to the third embodiment of the present invention.

[0052]FIGS. 19A to 19C are diagrams illustrating orientation of liquidcrystal molecules in a liquid crystal layer of an LCD according to afourth embodiment of the present invention.

[0053]FIG. 20 is a diagram showing response characteristics of the LCDaccording to the fourth embodiment of the present invention.

[0054]FIG. 21 is a schematic diagram showing the structure of a drivingcircuit of a conventional LCD.

[0055]FIG. 22 is a signal waveform chart illustrating how the responsecharacteristics are improved with the driving circuit shown in FIG. 21.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0056] (Embodiment 1)

[0057] Hereinafter, an embodiment of an LCD according to a first aspectof the present invention will be described with reference to theaccompanying drawings. The present embodiment is herein exemplarilydescribed regarding an NW mode LCD. However, the LCD according to thefirst aspect of the present invention is not limited to the NW mode LCD.

[0058] Functions of the LCD according to the first aspect of the presentinvention will now be described.

[0059] A liquid crystal panel of the LCD according to the first aspectof the present invention exhibits, in its V-T characteristics, anextreme transmittance at a voltage equal to or lower than the lowestgray-level voltage. An overshoot gray-level voltage is applied to theliquid crystal panel. Note that, the LCD is generally an AC-drivedevice, but the V-T characteristics thereof represent the relationbetween the absolute value of the voltage applied to the liquid crystallayer and the transmittance, based on a potential of the counterelectrode.

[0060] In the specification, a voltage applied to the liquid crystallayer for display on the LCD is referred to as a gray-level voltage Vg,and the gray-level voltage Vg is herein denoted corresponding to thegray level of the display. For example, for 64-gray-scale display fromzero (black) to 63 (white) gray levels, the gray-level voltage Vg fordisplay of zero gray level is denoted with V0, and the gray-levelvoltage Vg for display of 63 gray level is denoted with V63. In the NWmode LCD exemplified in the embodiment, V0 is the highest gray-levelvoltage, and V63 is the lowest gray-level voltage. In contrast, in theNB mode LCD, V0 is the lowest gray-level voltage, and V63 is the highestgray-level voltage.

[0061] Hereinafter, a signal that provides image information to bedisplayed on the LCD is referred to as an input image signal S, and avoltage that is applied to a picture element according to acorresponding input image signal S is referred to as a gray-levelvoltage Vg. The input image signals of 64 gray levels (S0 to S63)correspond to the respective gray-level voltages (V0 to V63). However,the correspondence between the input image signal S (gray-level data)and the gray-level voltage Vg in the NW mode is opposite to that in theNB mode. The gray-level voltage Vg is set so that a transmittance(display state) corresponding to the respective input image signal S isattained when the liquid crystal layer receiving the respectivegray-level voltage Vg reaches a steady state. This transmittance isreferred to as a steady-state transmittance. It should be understoodthat the values of the gray-level voltages VO to V63 may be varieddepending on the LCDs.

[0062] For example, the LCD is driven by an interlace driving method, sothat a single frame corresponding to a single image is divided into twofields, and a gray-level voltage Vg corresponding to the input imagesignal S is applied every field to the display section. It should beunderstood that a single frame may be divided into three or more fields,and the LCD may be driven by a non-interlace driving method. In thenon-interlace driving, a gray-level voltage Vg corresponding to theinput image signal S is applied every frame to the display section. Asingle field in the interlace driving or a single frame in thenon-interlace driving is herein referred to as a single vertical period.

[0063] The overshoot voltage is detected based on the comparison betweenthe respective input image signals S of the previous vertical period(immediately preceding vertical period) and the current vertical period.More specifically, in the case where the gray-level voltage Vgcorresponding to the input image signal S of the current vertical periodis lower than that corresponding to the input image signal S of theprevious vertical period, the overshoot voltage refers to a voltage thatis further lower than the gray-level voltage Vg corresponding to theinput image signal of the current vertical period. On the contrary, inthe case where the gray-level voltage Vg corresponding to the inputimage signal S of the current vertical period is higher than thatcorresponding to the input image signal S of the previous verticalperiod, the overshoot voltage refers to a voltage that is further higherthan the gray-level voltage Vg corresponding to the input image signal Sof the current vertical period.

[0064] The comparison of the input image signal S for detecting theovershoot voltage is made between the respective input image signals Sof the previous vertical period and the current vertical period forevery picture element. Even in the interlace driving in which imageinformation corresponding to a single frame is divided into a pluralityof fields, the input image signal S of a picture element of interest inthe previous frame and the input image signals S of the upper and lowerlines are used as supplementary signals, so that the signalscorresponding to all the picture elements are applied within a singlevertical period. Thus, the input image signals S of the previous andcurrent fields are compared with each other.

[0065] The difference between an overshoot gray-level voltage Vg and aprescribed gray-level voltage (gray-level voltage corresponding to theinput image signal S of the current vertical period) Vg is herein alsoreferred to as an overshoot amount. In addition, the overshootgray-level voltage Vg is herein also referred to as an overshootvoltage. The overshoot voltage may either be another gray-level voltageVg having a prescribed overshoot amount with respect to the prescribedgray-level voltage Vg, or a voltage that is prepared in advanceexclusively for overshoot driving (hereinafter, such a voltage isreferred to as dedicated overshoot-driving voltage). At least a higherdedicated overshoot-driving voltage and a lower dedicatedovershoot-driving voltage are respectively prepared as voltagesovershooting the highest gray-level voltage (the gray-level voltagehaving the highest voltage value among the gray-level voltages) and thelowest gray-level voltage (the gray-level voltage having the lowestvoltage value among the gray-level voltages).

[0066] The liquid crystal panel of the LCD according to the first aspectof the present invention has, in its V-T characteristics, an extremetransmittance at a voltage equal to or lower than the lowest gray-levelvoltage.

[0067] It is now assumed that the liquid crystal panel has an extremetransmittance at the lowest gray-level voltage. In this case, when avoltage overshooting the lowest gray-level voltage (lower dedicatedovershoot-driving voltage) is applied, the transmittance goes through avalue corresponding to the lowest gray-level voltage (in the NW mode,this value is the highest value among the transmittances used fordisplay, and corresponds to the extreme transmittance, and in the NBmode, this value is the lowest value among the transmittances used fordisplay, and corresponds to the extreme transmittance), and then reachesa value corresponding to the overshoot voltage (in the NW mode, thisvalue is a lower transmittance, and in the NB mode, is a highertransmittance).

[0068] It is assumed that the lowest gray-level voltage is set to avalue higher than the voltage corresponding to the extremetransmittance, and the voltage overshooting the lowest gray-levelvoltage (lower dedicated overshoot-driving voltage) is set to a valuelower than the voltage corresponding to the extreme transmittance. Whenthis lower dedicated overshoot-driving voltage is applied, thetransmittance goes through a value corresponding to the lowestgray-level voltage (in the NW mode, this value is the highest valueamong the transmittances used for display, and in the NB mode, is thelowest value among the transmittances used for display), and through theextreme value, and then reaches a value corresponding to the overshootvoltage (in the NW mode, this value is a lower transmittance, and in theNB mode, is a higher transmittance).

[0069] It is assumed that the lowest gray-level voltage is set to avalue higher than the voltage corresponding to the extremetransmittance, and the voltage overshooting the lowest gray-levelvoltage (lower dedicated overshoot-driving voltage) is set to a valueequal to or higher than the voltage corresponding to the extremetransmittance. When this lower dedicated overshoot-driving voltage isapplied, the transmittance goes through a value corresponding to thelowest gray-level voltage (in the NW mode, this value is the highestvalue among the transmittances used for display, and in the NB mode, isthe lowest value among the transmittances used for display), and thenreaches a value corresponding to the overshoot voltage (in the NW mode,this value is a higher transmittance, and in the NB mode, is a lowertransmittance).

[0070] The response time required for a fall (to the steady state) isalmost the same both in the case of applying the lowest gray-levelvoltage and applying the overshoot voltage. Therefore, application ofthe overshoot voltage can reduce the time for the transmittance to reacha value corresponding to the lowest gray-level voltage. In other words,in a liquid crystal panel that exhibits an extreme transmittance at avoltage equal to or lower than the lowest gray-level voltage, the liquidcrystal molecules in the liquid crystal layer with application of thelowest gray-level voltage has a substantially different orientationstate from that without application of a voltage. Therefore, furtherrelaxation is possible. Thus, the transmittance changes more steeplywith time as compared to the case of overshoot-driving a liquid crystalpanel having such V-T characteristics that exhibit a constanttransmittance (i.e., having no extreme value) over the voltage range ofthe lowest gray-level voltage or less (See FIGS. 5A and 5B).

[0071] Therefore, in the LCD according to the first aspect of thepresent invention, the fall response characteristics of the LCD can beimproved over the conventional overshoot driving. Note that, even if aliquid crystal panel that exhibits no extreme transmittance in the lowervoltage range is used, it is possible to improve the fall responsecharacteristics by setting the lowest gray-level voltage to a value thatis somewhat higher than the voltage corresponding to the highesttransmittance (NW mode) or the lowest transmittance (NB mode). However,such a somewhat higher lowest gray-level voltage reduces thetransmittance range available for the display. In contrast, in the LCDaccording to the first aspect of the present invention, the lowestgray-level voltage is set to a value equal to or higher than the voltagecorresponding to an extreme transmittance (maximal transmittance (NWmode) or minimal transmittance (NB mode)). Accordingly, the fallresponse speed can be improved while suppressing or preventing thetransmittance loss.

[0072] Particularly in the case where the lowest gray-level voltage isset to a value corresponding to the extreme transmittance, there is notransmittance loss. Note that, in order to enhance the effect ofimproving the response speed, it is preferable to set the lowestgray-level voltage to a value higher than that corresponding to theextreme transmittance. Even if the lowest gray-level voltage is set assuch, the transmittance loss can be reduced as compared to the case ofthe liquid crystal panel exhibiting no extreme value in the lowervoltage range. The reason for this is as follows: in the LCD accordingto the first aspect of the present invention, the liquid crystal layerwith application of the voltage corresponding to the extremetransmittance has a substantially different orientation state from thatwithout application of a voltage. Therefore, further relaxation ispossible. Thus, the relaxation phenomenon from the extreme transmittanceto the transmittance without application of the voltage can be utilizedfor the fall response.

[0073] It should be understood that the rise response speed of theliquid crystal layer increases as the applied voltage value is higher.Therefore the rise response characteristics can also be improved byapplication of an overshoot voltage.

[0074] Note that the liquid crystal panel that exhibits, in its V-Tcharacteristics, an extreme transmittance at a voltage equal to or lowerthan the lowest gray-level voltage is implemented by, e.g., adjustingthe retardation of the liquid crystal panel.

[0075] Unless otherwise specified, in the NW mode, “retardation of theliquid crystal panel” as used in the specification means the sum of aretardation of the liquid crystal layer in the state where a voltage isnot applied and a retardation of a phase compensator, and indicates theretardation to the light incident vertically to the display plane of theliquid crystal panel (which is in parallel with the plane of the liquidcrystal layer). It should be understood that, in the structure includingno phase compensator, the retardation of the liquid crystal panelcorresponds to the retardation of the liquid crystal layer in the statewhere a voltage is not applied. In the NB mode, “retardation of theliquid crystal panel” means the sum of the retardation of the liquidcrystal layer in the state where the maximum possible voltage for thedisplay is applied and the retardation of a phase compensator, andindicates the retardation to the light incident vertically to thedisplay plane of the liquid crystal panel. In the structure including nophase compensator, the retardation of the liquid crystal panelcorresponds to the retardation of the liquid crystal layer in the statewhere the maximum possible voltage for the display is applied. Theretardation of the liquid crystal layer is the difference (Δn) betweenthe maximum and minimum refractive indices of a liquid crystal materialmultiplied by the thickness (d) of the liquid crystal layer.

[0076] In general, the retardation of a transmission-type liquid crystalpanel is set so as to change in the range of about 260 nm in response toapplication of a gray-level voltage. In other words, the retardation ofthe liquid crystal panel is set so that the difference in retardation ofthe liquid crystal panel between the lowest- and highest-gray-leveldisplay states is about 260 nm. This is determined so as to increase thecontrast ratio for the green light having the highest human eye's colorsensitivity (i.e., the light having a wavelength of about 550 nm), aswell as in view of the display characteristics (viewing-angledependency) for the other colors. Depending on the specification of theLCD, the retardation is set within the range of about 250 nm to about270 nm. Hereinafter, “260 nm” is used as a typical preset retardationvalue.

[0077] Since the orientation state of the liquid crystal moleculeschanges in response to a voltage, the retardation of the liquid crystallayer changes according to the voltage. However, the liquid crystallayer has a layer anchored at the substrate surface, i.e., a layer whoseorientation state does not change in response to application of avoltage (in the voltage range used for normal display) (hereinafter,such a layer is referred to as “anchoring layer”). The retardation ofthe anchoring layer is about 40 nm to about 80 nm. Accordingly, theoverall retardation of the liquid crystal layer is the retardation ofthe anchoring layer added to the aforementioned preset value (about 260nm) (about 300 nm to about 340 nm).

[0078] A phase compensator for compensating for the retardation of theanchoring layer (e.g., a phase plate or phase film) may be provided.More specifically, a phase compensator may be provided which makes thetotal retardation of the liquid crystal layer and the phase compensatorequal to the aforementioned preset value (about 260 nm).

[0079] In the LCD according to the first aspect of the presentinvention, it is preferable that the difference in retardation of theliquid crystal panel between the states where no voltage is applied andwhere the highest gray-level voltage is applied (hereinafter, such adifference is also simply referred to as “the retardation difference ofthe liquid crystal panel”) is 300 nm or more. Provided that theretardation of the liquid crystal panel is set so as to change by 300 nmor more throughout the voltage range up to the highest gray-levelvoltage, about 260 nm can be ensured as a retardation range used fordisplay, and also the V-T characteristics that provide an extremetransmittance at a voltage equal to or lower than the lowest gray-levelvoltage can be implemented. It should be understood that, in thestructure making much account of the response speed, the retardationrange used for display may be reduced.

[0080] The effect of improving the fall response characteristics of theLCD according to the first aspect of the present invention can beobserved particularly in the NW mode liquid crystal panel. Therefore, itis preferable to apply the present invention to the NW mode LCD. In thecase where the present invention is applied to an NB mode liquid crystalpanel including a horizontal orientation liquid crystal layer and alsousing a phase compensator, an extreme (minimal) transmittance appears inthe black display, and therefore is not likely to be observed. Moreover,around the extreme transmittance in the black display, even a slightdifference in gray-level voltage results in a large difference in aretardation value. Therefore, it is difficult to compensate for thephase difference so as to provide excellent black display. In the casewhere the present invention is applied to an NB mode liquid crystalpanel including a vertical orientation liquid crystal layer, no extremetransmittance is observed in the black display. Therefore, the effect ofreducing the response time is not obtained.

[0081] Moreover, a parallel-orientation (homogeneous-orientation) liquidcrystal layer has a faster response speed (e.g., response time of about17 msec) than that of a twisted orientation liquid crystal layer and avertical orientation liquid crystal layer. Therefore, by applying theLCD according to the first aspect of the present invention to theparallel-orientation liquid crystal layer, further improvement inresponse speed is obtained, making it possible to implement an LCDhaving particularly excellent moving picture display characteristics(e.g., response time of about 10 msec or less).

[0082] (Retardation)

[0083] The NW mode liquid crystal panel included in the LCD of thepresent embodiment is adjusted in retardation so as to exhibit, in itsV-T characteristics, the maximal (and highest) transmittance at avoltage equal to or lower than the lowest gray-level voltage. Typically,the liquid crystal panel is set such that the retardation changes in therange of 300 nm or more in response to application of a voltage.

[0084] The reason for this will be described with reference to FIGS. 1,2A and 2B.

[0085] A V-T curve of the liquid crystal panel that includes aparallel-orientation liquid crystal layer including a liquid crystalmaterial with a positive refractive index anisotropy (Δn=n//−n⊥>0) isshown in FIG. 1. FIG. 1 also shows V-T curves of the liquid crystalpanels having different retardations. FIG. 2A shows avoltage-retardation curve of the liquid crystal panel having aretardation of 260 nm, and FIG. 2B shows a voltage-retardation curve ofthe liquid crystal panel having a retardation of 300 nm. In the graphsshowing the curves representing the transmittance or retardationchanging according to an applied voltage, the ordinate indicates arelative value (arbitrary unit) of the transmittance or retardation,regarding the lowest transmittance or retardation as zero. Accordingly,these graphs show a variation in transmittance or retardation accordingto a change in applied voltage.

[0086] The liquid crystal panels having various retardations shown inFIG. 1 can be obtained by using liquid crystal materials havingdifferent values An and/or by changing the thickness d of the liquidcrystal layer. The retardation value can also be adjusted using a phasecompensator.

[0087] First, regarding the liquid crystal layer with the anchoringlayer removed, the relation between the alignment state of the liquidcrystal molecules and the retardation will be described. When thevoltage is applied to the parallel-orientation liquid crystal layer, theliquid crystal molecules are raised (tilted) with respect to the surfaceof the liquid crystal layer), so that the maximum refractive index forthe light incident vertically to the liquid crystal layer becomessmaller than n// (the minimum refractive index is retained at n⊥).Accordingly, as shown in FIGS. 2A and 2B, the retardation is reducedupon application of the voltage. When the applied voltage is increased(a voltage equal to or higher than the saturation voltage is applied),the liquid crystal molecules are oriented vertically to the surface ofthe liquid crystal layer. Therefore, both the maximum and minimumrefractive indices of the liquid crystal layer become equal to n⊥, sothat the retardation is reduced to zero. However, since an actual liquidcrystal layer has an anchoring layer, the retardation is not reduced tozero. FIGS. 2A and 2B each shows a voltage-retardation curve of theliquid crystal panel provided with a phase compensator for compensatingfor the retardation of the anchoring layer. Herein, the retardation ofthe liquid crystal layer at an applied voltage of 5 V is cancelled.

[0088] In general, the liquid crystal panel is set to have the highesttransmittance when the retardation thereof is about 260 nm (250 to 270nm). Accordingly, in the case where the retardation without voltageapplication is about 260 nm or less (see the curves of 220 nm and 260 nmin FIG. 1), the transmittance gradually monotonously reduces withincrease in voltage from the state where the voltage is not applied. Incontrast, in the case where the retardation without voltage applicationexceeds about 260 nm (see the curves of 300 nm, 320 nm, 340 nm and 380nm in FIG. 1), the transmittance first gradually increases (until theretardation reaches about 260 nm) and then reduces with increase involtage.

[0089] Since the retardation of the liquid crystal panel (variationcaused by the voltage) is set to 300 nm or more, the transmittancereaches the highest (maximal) value at the applied voltage to the liquidcrystal layer higher than 0 V. Thus, the lowest gray-level voltage Vg(e.g., V63) is set to a value equal to or higher than this voltage, andalso a voltage lower than this voltage is applied as an overshootvoltage, so that the overshoot toward a lower voltage can be effectivelyconducted.

[0090] (Dedicated Overshoot-Driving Voltage and Gray-Level Voltage)

[0091] In the NW mode, the lowest gray-level voltage Vg of the LCDaccording to the first aspect of the present invention is set to a valueequal to or higher than the voltage corresponding to the highest steadytransmittance. The highest gray-level voltage Vg is set to a value equalto or lower than the voltage corresponding to the lowest steadytransmittance. Note that, in the NB mode, the lowest gray-level voltageVg is set to a value equal to or higher than the voltage correspondingto the lowest steady transmittance, and the highest gray-level voltageVg is set to a value equal to or lower than the voltage corresponding tothe highest steady transmittance.

[0092] The LCD according to the first aspect of the present inventionhas a retardation difference of, e.g., about 300 nm or more. Therefore,as shown in FIG. 1, the voltage corresponding to the highesttransmittance in the V-T curve of the NW mode LCD is a voltage thatprovides an extreme value. Thus, if the gray-level voltage Vg is set tothe range including a voltage lower than the voltage providing theextreme value, the transmittance is inversed, whereby gray-levelinversion is observed. In order to prevent this gray-level inversion,the lowest gray-level voltage is set to a value equal to or higher thanthe voltage providing the extreme value. It should be appreciated thatthe highest gray-level voltage Vg is set so as not to exceed thewithstand voltage of a driving circuit (a driver, and typically a driverIC (Integrated Circuit)).

[0093] In the LCD according to the first aspect of the presentinvention, a dedicated overshoot-driving voltage Vos is preset inaddition to the gray-level voltage Vg (V0 to V63). The dedicatedovershoot-driving voltage Vos includes a voltage Vos(L) lower than thegray-level voltage Vg and a voltage Vos(H) higher than the gray-levelvoltage Vg. A plurality of voltage values may be prepared for each ofVos(L) and Vos(H). The higher dedicated overshoot-driving voltage Vos(H)(the highest value if a plurality of voltages Vos(H) are prepared) isset so as not to exceed the withstand voltage of the driving circuit.The dedicated overshoot-driving voltage Vos is set such that the voltageVos combined with the gray-level voltage Vg (V0 to V63) does not exceedthe number of bits of the driving circuit.

[0094] Hereinafter, setting of the dedicated overshoot-driving voltageVos and the gray-level voltage Vg will be specifically described withreference to FIG. 3. FIG. 3 shows the relation between a V-T curve,dedicated overshoot-driving voltage Vos and gray-level voltage Vg. Thegray-level voltage Vg (V0 (black) to V63) is set within the range fromthe voltage corresponding to the highest transmittance to the voltagecorresponding to the lowest transmittance. The lower dedicatedovershoot-driving voltage Vos(L) (e.g., 32 gray levels Vos(L)1 toVos(L)32) is set within the range from 0 V to a voltage lower than V63(the lowest gray-level voltage Vg). The higher dedicatedovershoot-driving voltage Vos(H) (e.g., 32 gray levels Vos(H)1 toVos(H)32) is set within the range from a voltage higher than V0 (thehighest gray-level voltage Vg) to a voltage that does not exceed thewithstand voltage of the drive circuit. Note that the number of graylevels of the gray-level voltage Vg as well as the number of gray levelsof the dedicated overshoot-driving voltage Vos can be set arbitrarily soas not to exceed the number of bits of the driving circuit. The numberof gray levels of the lower dedicated overshoot-driving voltage Vos(L)may be different from that of the higher dedicated overshoot-drivingvoltage VOS(H).

[0095] The voltage applied to conduct the overshoot driving ispredetermined corresponding to a change in input image signal S, andeither the gray-level voltage Vg or the dedicated overshoot-drivingvoltage Vos is used.

[0096] For example, in the case where the gray-level voltage Vgcorresponding to the input image signal S of the current field is lowerthan that corresponding to the input image signal S of the previousfield, a voltage that is lower than the gray-level voltage Vgcorresponding to the input image signal S of the current field isselected from the gray-level voltage Vg and the lower dedicatedovershoot-driving voltage Vos(L), and applied to the liquid crystalpanel. A voltage used for overshoot driving is predetermined so as toattain a steady state transmittance corresponding to the input imagesignal S of the current field within a predetermined time (e.g., 16.7msec) from application of the voltage of the current field.Alternatively, the voltage used for overshoot driving is predeterminedso as to attain such a transmittance that does not provide uniformdisplay when visually observed.

[0097] The voltage used for overshoot driving is determined for acombination of the input image signal S (e.g., 64 gray levels) of theprevious field and the input image signal S of the current field (64gray levels) (however, this voltage is not necessary for the combinationhaving no change in gray level). Depending on the response speed of theliquid crystal panel, there may be a combination of the gray levels thatdoes not require the overshoot driving. The number of gray levels of thededicated overshoot-drive voltage Vos may also be varied as appropriate.

[0098] (Circuit for Conducting Overshoot Driving)

[0099] The structure of a driving circuit 10 in the LCD of the presentembodiment will now be described with reference to FIG. 4.

[0100] The driving circuit 10 receives an external input image signal S,and supplies a corresponding driving voltage to a liquid crystal panel15. The driving circuit 10 includes an image storage circuit 11, acombination detection circuit 12, an overshoot voltage detection circuit13, and a polarity inversion circuit 14.

[0101] The image storage circuit 11 retains at least one field image ofthe input image signal S. It should be understood that, in the casewhere a single frame is not divided into a plurality of fields, theimage storage circuit 11 retains at least one frame image. Thecombination detection circuit 12 compares the input image signal S ofthe current field with the input image signal S of the previous fieldretained in the image storage circuit 11, and outputs a signalindicating that combination to the overshoot voltage detection circuit13. The overshoot voltage detection circuit 13 detects a driving voltagecorresponding to the combination detected by the combination detectioncircuit 12, from the gray-level voltage Vg and the dedicatedovershoot-drive voltage Vos. The polarity inversion circuit 14 convertsthe driving voltage detected by the overshoot voltage detection circuit13 into an AC signal for supply to the liquid crystal panel (displaysection) 15.

[0102] Hereinafter, the input/output signal of each circuit will bedescribed. In the following description, it is assumed that a voltageused for fall overshoot driving is preset to a gray-level voltage Vgthat is lower than the gray-level voltage Vg corresponding to the inputimage signal S.

[0103] First, the image storage circuit 11 retains the input imagesignal S corresponding to one field before the input image signal S ofthe current field.

[0104] Then, the combination detection circuit 12 detects, for everypicture element, a combination of the current input image signal S andthe input image signal S of the previous field retained in the imagestorage circuit 11. For example, for a given picture element, thecombination detection circuit 12 detects a combination (S20, S40) of theinput image signal S20 of the previous field and the input image signalS40 of the current field.

[0105] The overshoot voltage detection circuit 13 detects a gray-levelvoltage V60 (corresponding to an input image signal S60) that ispredetermined for the combination (S20, S40) detected by the combinationdetection circuit 12, and supplies the gray-level voltage V60 to thepolarity inversion circuit 14 as a driving voltage. This operationcorresponds to conversion of the input image signal S40 of the currentfield into S60. For example, the process of detecting the gray-levelvoltage V60 as a predetermined overshoot voltage corresponding to thecombination (S20, S40) detected by the combination detection circuit 12may be conducted either by a lookup table method or by performing apredetermined operation.

[0106] Finally, the polarity inversion circuit 14 converts thegray-level voltage V60 to an AC signal for supply to the liquid crystalpanel 15.

[0107] Hereinafter, the operation of conducting the overshoot drivingusing the dedicated overshoot-driving voltage Vos in the LCD of thepresent embodiment will be described.

[0108] For example, for a 64-gray-level (6-bit) input image signal S,the overshoot voltage detection circuit 13 can detect a driving voltagefor prescribed overshoot driving, from 7 bits (64 gray-level voltages Vg(V0 to V63) and 64 overshoot voltages Vos (higher voltages: Vos(H)1 toVos(H)32; and lower voltages: Vos(L)1 to Vos(L)32).

[0109] This will be specifically described for a fall. It is now assumedthat the input image signal S40 is shifted to S63 after one field. Theinput image signal S40 is retained in the image storage circuit 11. Thecombination detection circuit 12 detects the combination (S40, S63).Then, the overshoot voltage detection circuit 13 detects a dedicatedovershoot-driving voltage Vos(L)20 predetermined so as to attain asteady transmittance corresponding to the input image signal S63 withinone field, and supplies the voltage Vos(L)20 to the polarity inversioncircuit 14 as a driving voltage. This voltage Vos(L)20 is converted intoan AC signal by the polarity inversion circuit 14 and then supplied tothe liquid crystal panel.

[0110] The above operation corresponds to conversion of a 6-bit digitalinput image signal S into a 7-bit digital input image signal S includinga dedicated overshoot-driving voltage Vos (64 gray levels) by theovershoot voltage detection circuit 13.

[0111] Note that, when there is no change between the input imagesignals S, an overshoot driving voltage is not applied. For example,when the combination detection circuit 12 detects the combination (S40,S40), the overshoot voltage detection circuit 13 outputs a gray-levelvoltage V40 corresponding to S40 to the polarity inversion circuit 14 asa driving voltage.

[0112] A field to be subjected to the aforementioned overshoot drivingis not limited to the first field to which the input image signal s isshifted. In addition to the first field, the following field or thefield after the following field may be subjected to the overshootdriving. Such a driving method may be conducted with a combination ofappropriate circuits. Note that, in the case where a single frame isdivided into a plurality of fields for driving, it is preferable thatthe first field or all the fields are subjected to the overshootdriving. Moreover, in the case where a plurality of fields within asingle frame are subjected to the overshoot driving, the overshootamounts (that is, shift amounts from a predetermined gray-level voltageVg) used in the respective fields may be different from each other. Forexample, overshoot driving of the second field may be conducted with anovershoot amount smaller than that used in overshoot driving of thefirst field.

[0113] (Change in Transmittance in Overshoot Driving)

[0114] Hereinafter, response characteristics upon overshoot-driving theLCD of the present embodiment will be described with reference to FIGS.5A and 5B.

[0115]FIG. 5A shows the respective V-T curves of the LCD of the presentembodiment (liquid crystal panel with 320-nm retardation) and the LCD ofa comparative example (liquid crystal panel with 260-nm retardation).The liquid crystal panel of the present embodiment has an extreme valuein the V-T curve, whereas the liquid crystal panel of the comparativeexample does not have an extreme value in the V-T curve. The respectiveliquid crystal layers of these two liquid crystal panels have the samethickness, and the respective liquid crystal materials used therein havethe same dielectric anisotropy (Δε) and viscosity, and have differentvalues Δn. The retardation is adjusted with a phase compensator. Inthese liquid crystal panels, substantial change in retardation starts atthe same voltage (Vth). As the applied voltage is gradually increasedfrom a lower voltage, the transmittance of the 260-nm liquid crystalpanel decrease monotonously beyond Vth, whereas the transmittance of the320-nm liquid crystal panel first increases beyond Vth, reaches theextreme value and then degreases monotonously. In both liquid crystalpanels, the highest transmittance is T(c), and the steady transmittancefor the applied voltage V(a) is T(a).

[0116]FIG. 5B is a graph schematically showing a change in transmittancewith time in the LCD of the present embodiment. A time interval shown bythe dashed line in FIG. 5B corresponds to a single field. FIG. 5B showsa change from a first field of the black display (corresponding to thelowest gray level S0) to a second field of the white display(corresponding to the highest gray level S63). In FIG. 5B, thetransmittance attains a steady state at the same time ts. As describedbefore, this is because a fall in the LCD corresponds to the relaxationphenomenon of the orientation of the liquid crystal molecules.

[0117] Curve L1 in FIG. 5B shows the case where the voltage V(a), i.e.,a lower dedicated overshoot-driving voltage Vos, was applied to theliquid crystal panel with 320-nm retardation in the second field (thepresent invention). In contrast, curve L2 shows the case where thelowest gray-level voltage V(b) corresponding to the same steady-statetransmittance as in the case of the dedicated overshoot-driving voltageV(a) was applied to the liquid crystal panel with 320-nm retardation.For simplicity of comparison, the voltage corresponding to the sametransmittance as that of the lowest gray-level voltage V(b) was used asthe dedicated overshoot-driving voltage V(a). However, setting of thededicated overshoot-driving voltage V(a) is not limited to this.

[0118] As shown by curve L1, when the lower dedicated overshoot-drivingvoltage V(a) is applied, the transmittance first increases from thevalue of the first field, and then decreases toward the steady statetransmittance of the dedicated overshoot-driving voltage V(a), as longas a single field is long enough.

[0119] This is due to a change in retardation of the liquid crystalpanel of the present embodiment. In response to application of thededicated overshoot-driving voltage V(a), the liquid crystal moleculesfall toward the steady state. It should be appreciated that theretardation of the liquid crystal layer increases toward the steadystate corresponding to the applied dedicated overshoot-driving voltageV(a). More specifically, the retardation first increases, and stillincreases beyond 260 nm. Then, the retardation gets close to a steadyretardation corresponding to the applied dedicated overshoot-drivingvoltage V(a). In general, the retardation corresponding to the highesttransmittance is about 260 nm. Therefore, the transmittance firstincreases and then decreases, whereby the change in transmittance asdescribed above is obtained (see FIG. 5A).

[0120] On the other hand, as shown by curve L2, when merely the lowestgray-level voltage V(b) is applied instead of V(a) (i.e., when theovershoot driving is not conducted), the transmittance increased fromthe value of the first field toward the steady state transmittancecorresponding to the lowest gray-level voltage V(b). In response toapplication of the gray-level voltage V(b), the liquid crystal moleculesfall toward the steady state. It should be appreciated that theretardation increases toward the steady state of the applied voltageV(b). In this case, the retardation does not exceed about 260 nm (theretardation that provides an extreme transmittance). Therefore,reduction in transmittance does not occur.

[0121] Note that, when the voltage V(a) is applied to the liquid crystalpanel of 260-nm retardation, the response characteristics changeapproximately in the same manner as that of curve L2. When a voltage(overshoot voltage) that is even lower than V(a) (the lowest gray-levelvoltage) is applied to the liquid crystal panel of 260-nm retardation,the response time is further reduced but only to a small extent.Therefore, a steeper response curve than curve L1 is not obtained.

[0122] As can be appreciated from the above, in the case where thededicated overshoot-driving voltage V(a) is applied to a liquid crystalpanel having a retardation of 300 nm or more, the transmittanceincreases extremely steeply in the second field, as shown by curve L1.According to the present embodiment, the fall response characteristicsare improved by utilizing such a steep change in transmittance, wherebyan LCD preferably used for moving picture display is provided.

[0123] Hereinafter, response characteristics of the LCD of the presentembodiment (liquid crystal panel with 300-nm retardation) will bedescribed with reference to FIG. 5C. As shown in FIG. 5C, for this LCD,the lowest gray-level voltage was set to a voltage (v(c)) correspondingto the highest transmittance (T(c)), and overshoot driving was conducted(a voltage (V(d)) was applied). For comparison, response characteristicsof a liquid crystal panel that does not have an extreme value in its V-Tcurve (liquid crystal panel with 260-nm retardation) are also described.For this liquid crystal panel, the lowest gray-level voltage was set toa voltage (V(d)) corresponding to the highest transmittance (T(c)), andovershoot driving was conducted (a voltage V(d′) was applied).

[0124]FIG. 5D shows response curves L3 and L4 of the liquid crystalpanel with 320-nm retardation. Response curve L3 shows the case wherethe lowest gray-level voltage was set to the voltage (V(c))corresponding to the highest transmittance (T(c)), and overshoot drivingwas conducted (the voltage (V(d)) was applied). Response curve L4 showsthe case where the lowest gray-level voltage V(c) was applied withoutconducting the overshoot driving.

[0125] As is apparent from the comparison between curves L3 and L4 ofFIG. 5D, even when the lowest gray-level voltage is set to the voltageV(c) corresponding to the highest transmittance in the liquid crystalpanel with 320-nm retardation, the fall response characteristics can beimproved by application of the overshoot voltage V(d), as in the casedescribed above in connection with FIG. 5B. The reason for this is asfollows: in the V-T curve of the 320-nm liquid crystal panel, the pointthat provides the highest transmittance is a maximal value, and afurther change in retardation, i.e., further relaxation of orientationof the liquid crystal molecules, is still possible in the voltage rangelower than V(c). However, an application period of the overshoot voltageV(d) must be adjusted so that the transmittance does not decrease fromthe highest value.

[0126] Note that, as described above, setting the lowest gray-levelvoltage to the voltage V(c) corresponding to the highest transmittanceallows the response characteristics to be improved without sacrificingthe transmittance. However, a greater effect of improving the responsecharacteristics is obtained when the lowest gray-level voltage is set toa value higher than the voltage corresponding to the extremetransmittance, as shown in FIG. 5B. Accordingly, depending onapplications of the LCD, and the like, the lowest gray-level voltage canbe set to a value equal to or higher than the voltage corresponding tothe extreme transmittance.

[0127] On the other hand, as shown in FIG. 5C, when the lowestgray-level voltage is set to the voltage providing the highesttransmittance in the liquid crystal panel with 260-nm retardation, theresponse characteristics cannot be improved even by application of thededicated overshoot-driving voltage V(d′) less than the lowestgray-level voltage. In other words, whether the lowest gray-levelvoltage V(d) or the overshoot voltage V(d′) is applied, the resultantresponse curve is approximately the same as curve L4 of FIG. 5D. Thereason for this is as follows: as described before, in the flat portionof the 260-nm curve, the liquid crystal molecules have substantially thesame orientation state and thus have the same restoring force.Accordingly, in order to improve the fall response characteristics ofthe liquid crystal panel with 260-nm retardation, the lowest gray-levelvoltage must be set to a value (e.g., V(c)) higher than the voltagecorresponding to the highest transmittance, sacrificing thetransmittance. An increased response speed by the overshoot driving(e.g., application of V(d)) can be achieved only by setting the lowestgray-level voltage as such.

[0128] As described above, according to the present embodiment, an LCDhaving improved fall response characteristics and preferably used formoving picture display is provided.

[0129] The above example has been described for the liquid crystal panelthat includes a liquid crystal layer having a relatively high responsespeed, i.e., the liquid crystal panel achieving a steady-statetransmittance corresponding to an applied voltage within a single field.However, in a liquid crystal panel that requires a relatively long time(e.g., two fields) to reach a steady-state transmittance correspondingto an applied voltage, a prescribed display state (transmittance) cannotbe implemented with the response characteristics shown by curve L2. Incontrast, with the response characteristics of curve L1, a prescribeddisplay state can be implemented in a single field, as shown in FIG. 6.FIG. 6 shows the time-axis unit of FIG. 5B reduced by half. As a result,blurred moving picture display is prevented from being produced byoverlapping of the respective images of the previous field and thecurrent field.

[0130] Alternatively, in the case where the overshoot driving isconducted to a liquid crystal panel that includes a liquid crystal layerhaving a relatively high response speed as shown in FIG. 5B, theresponse characteristics shown in FIG. 6 can also be obtained by thefollowing method: a field of FIG. 5B is further divided into two fields,so that the overshoot-drive voltage V(a) is applied in the former fieldand the voltage V(b) corresponding to a prescribed gray-level voltage Vgis applied in the latter field. In other words, by doubling a frequencyfor supplying a driving voltage to the liquid crystal panel, thetransmittance is prevented from decreasing after increasing to aprescribed value or more as shown by curve L1 of FIG. 5B, and anextremely steep change in transmittance can be implemented as shown inFIG. 6. Thus, by further improving the response characteristics of theliquid crystal panel that attains a steady-state transmittancecorresponding to an applied voltage within a single field even withoutconducting the overshoot driving, the time for the liquid crystal panelto be in a predetermined display state (time integral value of thetransmittance) is increased, whereby the display quality (brightness,contrast ratio and the like) can be improved.

[0131] Thus, according to the present invention, a fast-response LCDsuitable for moving picture display can be obtained.

[0132] (Display Mode)

[0133] The present invention is applicable to various LCDs. As describedabove, however, the response characteristics of the liquid crystal paneldepend on the response speed of the liquid crystal layer (liquid crystalmaterial, orientation mode and the like). Accordingly, by using a liquidcrystal layer having a high response speed, a faster LCD havingexcellent moving picture display characteristics can be obtained.

[0134]FIG. 7 schematically shows a NW-mode transmission-type liquidcrystal panel 20 in ECB (Electrically Controlled Birefringence) modeusing a parallel-orientation (homogeneous-orientation) liquid crystallayer. The ECB mode is known as a liquid crystal mode having a fastresponse speed.

[0135] The liquid crystal panel 20 includes a liquid crystal cell 20 a,a pair of polarizers 25 and 26 interposing the liquid crystal cell 20 atherebetween, and phase compensators 23 and 24 provided between therespective polarizers 25, 26 and the liquid crystal cell 20 a.

[0136] The liquid crystal cell 20 a includes a liquid crystal layer 27provided between a pair of substrates 21 and 22. The substrates 21 and22 each includes a transparent substrate (e.g., glass substrate), atransparent electrode (not shown) for applying a voltage to the liquidcrystal layer 27, and an alignment film (not shown) for defining theorientation direction of liquid crystal molecules 27 a in the liquidcrystal layer 27. The transparent electrode and the alignment film areboth provided at the surface of the transparent substrate that faces theliquid crystal layer 27. It should be understood that a color filterlayer (not shown) may further be included as required. The transparentelectrode is formed from, e.g., ITO (Indium Tin Oxide).

[0137] The liquid crystal layer 27 is a parallel-orientation liquidcrystal layer. When a voltage is not applied, the liquid crystalmolecules 27 a in the liquid crystal layer 27 are oriented substantiallyin parallel with the plane of the liquid crystal layer 27 (in parallelwith the substrate surface) (but slightly tilted with respect to theplane by a pre-tilt angle), and also substantially in parallel with eachother (without being affected by the pre-tilt angle). An index ellipsoidof an anchoring layer is slightly tilted by the pre-tilt angle clockwiseabout the X-axis in the XYZ coordinate system having the plane of theliquid crystal layer 27 (i.e., the display plane) as XY plane.

[0138] The parallel-orientation liquid crystal layer is obtained byrubbing the alignment films provided on both sides of the liquid crystallayer 27 in anti-parallel with each other (see the arrows indicating therubbing directions in FIG. 7). Note that, if the alignment filmsprovided on both sides of the liquid crystal layer 27 are rubbed inparallel with each other, the liquid crystal molecules at one alignmentfilm make twice the pre-tilt angle with those at the other alignmentfilm. Therefore, the liquid crystal molecules 27 a are not oriented inparallel with each other.

[0139] The pair of polarizers (e.g., polarizing plates or polarizingfilms) 25 and 26 are provided such that their respective absorption axes(the arrows in FIG. 7) are orthogonal to each other and extend at anangle of 45 degrees with respect to the aforementioned rubbing direction(the orientation direction of the liquid crystal molecules within theplane of the liquid crystal layer).

[0140] As shown in FIG. 7, in each of the phase compensators (e.g.,phase plates or phase films) 23 and 24, an index ellipsoid (havingprincipal axes a, b and c) is slightly rotated about the a-axis, whichis in parallel with the X-axis, in the XYZ coordinate system having theplane of the liquid crystal layer 27 (i.e., the display plane) as XYplane. Herein, the Y-axis is in parallel (or anti-parallel) with therubbing direction, and the b-axis of the index ellipsoid is inclinedfrom the Y-axis. In other words, the major axis (b-axis) of the indexellipsoid is inclined counterclockwise with respect to the X-axis withinthe YZ plane. The phase compensators 23 and 24 thus provided arereferred to as inclined phase compensators.

[0141] These phase compensators 23 and 24 have a function to compensatefor the retardation of the anchoring layers of the liquid crystal layer27. Even if a voltage of, e.g., 7 V is applied to the liquid crystallayer 27, the liquid crystal molecules anchored by the alignment films(not shown) maintains their orientation in parallel with the plane ofthe liquid crystal layer 27. Therefore, the retardation of the liquidcrystal layer 27 does not become zero. The phase compensators 23 and 24compensate for (cancel) this retardation.

[0142] It is now assumed that, as a typical example, the principalrefractive indices na, nb and nc in the respective principal-axisdirections are given by the expression: na=nb>nc. As schematically shownin FIG. 8, when the index ellipsoids of the phase compensators 23 and 24have an inclination angle (an angle of the b-axis from the Y-axis) ofzero degree, the transverse (in-plane) retardation of the phasecompensators 23 and 24 (retardation for the light incident from thedirection normal to the display plane (in parallel with the Z-axis inthe figure)) is zero. However, as the inclination angle is increased,the retardation is produced and increased. This can be understood asfollows: as shown in FIG. 8, the index ellipsoid having an inclinationangle of zero degree looks like a perfect circle as viewed from thedirection normal to the display plane. However, as the inclination angleis increased, the index ellipsoid looks more like an ellipsoid.

[0143] Accordingly, when the phase compensators 23 and 24 each havingthe inclined index ellipsoid as described above are provided such thatthe inclination direction (b-axis direction) is in parallel oranti-parallel with the rubbing direction, retardation of the anchoringlayers can be cancelled by the transverse (in-plane) retardation of thephase compensators 23 and 24. Accordingly, in the above example, theretardation of the liquid crystal layer 27 at the applied voltage of 7 Vis cancelled (the retardation of the liquid crystal panel 20 at theapplied voltage of 7 V is reduced to zero), whereby the transmittance of0%, i.e., black display, can be implemented.

[0144] The transverse (in-plane) retardation of the phase compensators23 and 24 can be adjusted with the principal refractive indices,inclination angle, and thickness of the respective index ellipsoid. Bychanging the amount of transverse (in-plane) retardation of the phasecompensators 23 and 24, the amount of retardation of the liquid crystalpanel 20 a to be cancelled can be changed. Accordingly, not only theretardation of the anchoring layers of the liquid crystal layer 27 butalso the retardation of the liquid crystal layer 27 upon application ofa given voltage are cancelled, so that the range of the gray-levelvoltage Vg can be arbitrarily adjusted. For example, FIG. 9 shows V-Tcurves of various liquid crystal panels 20. In these liquid crystalpanels 20, the principal refractive indices and inclination angle of theindex ellipsoids are fixed, and only the thickness d of the phasecompensators 23 and 24 (thickness in the direction normal to the displayplane) are varied. Note that the transmittance is a transmittance in thedirection normal to the display plane. Thus, it can be appreciated thatthe V-T curve can be controlled by controlling the opticalcharacteristics of the phase compensators 23 and 24. It is apparent fromthe foregoing description that the same effects can also be obtained bycontrolling the inclination angle and/or principal refractive indices ofthe index ellipsoid.

[0145] The response time of the liquid crystal panel 20 (according tothe conventional driving method that does not use the overshoot driving)is about a half of 30 ms, which is a typical response time of theconventional TN mode liquid crystal panel. Although the liquid crystallayer of the TN mode liquid crystal panel has a twisted orientationstructure, the homogeneous orientation does not have a twistedorientation structure. Therefore, it can be understood that such a shortresponse time results from the simplicity of the orientation structure.

[0146] Moreover, an optical element for diffusing the light transmittedin or near the direction normal to the display plane (i.e., the displaylight) in the upward and downward directions with respect to the line ofsight of the viewer, that is, an optical element having the lens effectonly in a one-dimensional direction (e.g., BEF (Brightness EnhancementFilm) made by Sumitomo 3M Ltd.) is provided on the display plane of theliquid crystal panel 20. Thus, the liquid crystal panel 20 having nearlyconstant display quality regardless of the viewing angle, and thushaving an extremely wide viewing angle can be obtained.

[0147] The LCD 30 according to the present embodiment is schematicallyshown in FIG. 10.

[0148] The LCD 30 includes the liquid crystal panel 20 shown in FIG. 7and the driving circuit 10 shown in FIG. 4. The LCD 30 is a NW modetransmission-type LCD.

[0149] The liquid crystal panel 20 includes a thin film transistor (TFT)substrate 21 and a color filter substrate (hereinafter, referred to as“CF substrate”) 22. These substrates are both made by a known method.The LCD 30 of the present embodiment is not limited to the TFT-type LCD.However, an active-matrix LCD such as TFT- or MIM- (Metal InsulatorMetal) type LCD is preferable in order to implement a rapid responsespeed.

[0150] The TFT substrate 21 has picture-element electrodes 32 of ITOformed on a glass substrate 31, and an alignment film 33 formed over thesurface of the picture-element electrodes 32 that faces the liquidcrystal layer 27. The CF substrate 22 has a counter electrode (commonelectrode) 36 of ITO formed on a glass substrate 35 and an alignmentfilm 37 formed over the surface of the counter electrode 36 that facesthe liquid crystal layer 27. The alignment films 33 and 37 are formedfrom, e.g., polyvinyl alcohol or polyimide. Each alignment film 33, 37has its surface rubbed in one direction. The TFT substrate 21 and the CFsubstrate 22 are laminated together such that their respective rubbingdirections are in anti-parallel with each other. Then, a nematic liquidcrystal material having a positive dielectric anisotropy A E isintroduced therebetween, whereby the parallel-orientation liquid crystallayer 27 is obtained. It is herein assumed that the retardation of theliquid crystal layer 27 alone is 400 nm. The liquid crystal layer 27 issealed with a sealant 38.

[0151] The phase compensators 23 and 24 having a transverse (in-plane)retardation of 80 nm are laminated onto the respective outer surfaces ofthe TFT substrate 21 and CF substrate 22 such that the respective slowaxes of the phase compensators 23 and 24 are orthogonal to therespective rubbing direction. The overall retardation of the liquidcrystal panel 20 including the retardation of the phase compensators 23and 24 is 320 nm. The phase compensators 23 and 24 as well as thepolarizers 25 and 26 are arranged as described above in connection withFIG. 7.

[0152] The LCD 30 has V-T characteristics as shown by the 320-nm curveof FIG. 1. More specifically, the transmittance reaches the highest(maximal) value at the applied voltage of about 2 V, and then decreaseswith increase in applied voltage.

[0153] Hereinafter, the specific structure of the driving circuit 10will be described.

[0154] A 6-bit (64-gray-level) progressive signal at 60 Hz for one frameis used as an input image signal S. This input image signal S issequentially retained in the image storage circuit 11. Then, for everypicture element, the combination detection circuit 12 detects at 120 Hza combination of the current input image signal S and the input imagesignal S of the previous frame that is retained in the image storagecircuit 11. Herein, the combination detection circuit 12 detects thecombination at 120 Hz in order to conduct double-speed writing describedbelow. The input image signal S is a signal at 60 Hz for one frame.Therefore, the input image signal S is converted into a signal having adouble frequency (120 Hz) in an appropriate portion within the drivingcircuit 10. This conversion is herein conducted in the combinationdetection circuit 12.

[0155] From a 7-bit voltage (32 gray levels between the lower dedicatedovershoot-driving voltages 0 V and 2 V; 64 gray levels between thegray-level voltages 2.1 V and 5 V; and 32 gray levels between the higherdedicated overshoot-driving voltages 5.1 V and 6.5 V), the overshootvoltage detection circuit 13 detects a predetermined overshoot voltagecorresponding to the combination detected by the combination detectioncircuit 12. It is herein assumed that the overshoot voltage is a 120-Hzvoltage. This overshoot voltage is supplied to the polarity inversioncircuit 14 and converted into a 120-Hz AC voltage. This 120-Hz ACvoltage is supplied to the liquid crystal panel 20. In other words, the60-Hz input image signal S to the driving circuit 10 is output from thedriving circuit 10 to the liquid crystal panel 20 as a 120-Hz imagesignal. Accordingly, the input image signal S at 60 Hz for one frame isconverted into two fields of an output image signal at 120 Hz for onefield (hereinafter these two fields are referred to as “first and secondsub-fields”). Thus, double-speed writing to the liquid crystal panel 20is conducted.

[0156] Herein, the driving circuit 10 is set as follows: in response toa change in input image signal S (60 Hz), the driving circuit 10 outputsthe aforementioned overshoot voltage in the first sub-field of 120 Hz,and outputs a gray-level voltage Vg (no overshoot) corresponding to theinput image signal S of the current frame to the liquid crystal panel 20in the second sub-field.

[0157]FIG. 11 shows the response characteristics (solid line) of the LCD30 of the present embodiment. As a comparative example, FIG. 11 alsoshows the response characteristics (dashed line) obtained withoutconducting overshoot driving. FIG. 11 further shows the input imagesignal S, a voltage that is written at a double speed to the liquidcrystal panel 20, and a voltage that is output to the liquid crystalpanel without conducting the overshoot driving (without conductingdouble-speed driving either) in the comparative example.

[0158] As shown in FIG. 11, in the case where the input image signal (60Hz) changes toward a higher gray level (toward a lower voltage) from thefirst field to the second field, application of merely a prescribedgray-level voltage does not allow the transmittance to attain aprescribed value in the second field as shown by the dashed line. Incontrast, the overshoot driving allows the transmittance to attain aprescribed value in a ½ field (in a single sub-field) as shown by thesolid line. The effect of improving the response characteristicsaccording to the present invention can be obtained even when the inputimage signal S in the second field is a signal of the highest graylevel.

[0159] Note that the reason why the response characteristics of thecomparative example (dashed line) changes in a discontinuous manner isas follows: during a charge-retaining period of the liquid crystal layer27, the liquid crystal capacitance increases according to a change inliquid crystal orientation, so that the voltage being applied to theliquid crystal layer 27 is reduced.

[0160] Note that, in the description of the driving circuit 10, anon-interlace driven LCD in which a single frame corresponds to a singlevertical period has been described as the LCD of present embodiment.However, the LCD according to the first aspect of the present inventionis not limited to this, but can also be applied to an interlace-drivenLCD in which a single field corresponds to a single vertical period.

[0161] (Embodiment 2)

[0162] Hereinafter, an embodiment of the LCD according to a secondaspect of the present invention will be described with reference to thedrawings. However, the LCD according to the second aspect of the presentinvention is not limited to the following embodiment.

[0163]FIG. 12 schematically shows the structure of the LCD according tothe present embodiment. Note that, in the following embodiment, aninterlace-driven LCD in which a single field corresponds to a singlevertical period is exemplarily described.

[0164] In the case where the gray-level voltage Vg is referred to in theorder of magnitude, the gray-level voltage is denoted with Vv. Forexample, for 64-gray-scale display from zero (black) to 63 (white) graylevels, the gray-level voltage having the lowest value is denoted withVv0, and the gray-level voltage having the highest value is denoted withVv63. In the case of the NW mode LCD, Vv0 is a voltage for displayingthe highest gray level (63 gray level), and Vv63 is a voltage fordisplaying the lowest gray level (zero gray level). In contrast, in theNB mode LCD, Vv0 is a voltage for displaying the lowest gray level (zerogray level), and Vv63 is a voltage for displaying the highest gray level(63 gray level).

[0165] This LCD includes a liquid crystal panel 15 and a driving circuit10. The liquid crystal panel 15 has a plurality of picture-elementcapacitors Cpix arranged in a matrix, and TFTs 1 electrically connectedto the respective picture-element capacitors Cpix. Each TFT 1 has itsgate electrode 1G connected to a corresponding scanning line 2 and itssource electrode 1S connected to a corresponding signal line 3. Thedriving circuit 10 applies a scanning voltage and a driving voltage tothe scanning and source lines, respectively. Each TFT 1 has its drainelectrode ID connected to a corresponding picture-element capacitorCpix.

[0166] Each picture-element capacitor Cpix includes a liquid crystalcapacitor Clc and a storage capacitor Cs that is electrically connectedin parallel with the liquid crystal capacitor. Each liquid crystalcapacitor Clc is formed from a corresponding picture-element electrode,a counter electrode, and a liquid crystal layer provided therebetween.With a driving voltage supplied from the driving circuit 10 through acorresponding TFT 1, the picture-element capacitor Cpix is charged intoa charged state corresponding to an input image signal, so that thedisplay state is updated every field. Herein, the capacitance ratio ofthe storage capacitor Cs to the liquid crystal capacitor Clc(hereinafter, this ratio is also referred to as Cs/Clc for simplicity)is set to 1 or more (Cs/Clc.1). When at least the highest gray-levelvoltage is applied, the picture-element capacitor Cpix retains 90% ormore of the charging voltage over one field. In other words, by settingthe capacitance ratio of the storage capacitor Cs to the liquid crystalcapacitor Clc to Cs/Clc.1, the response speed (step responsecharacteristics) of the charging characteristics of the picture-elementcapacitor is improved. Accordingly, when at least the highest gray-levelvoltage is applied, the picture-element capacitor Cpix retains 90% ormore of the charging voltage over one field.

[0167] First, the storage capacitor Cs will be described.Conventionally, the storage capacitor Cs is generally provided in theTFT-type LCD. The storage capacitor Cs is connected in parallel with theliquid crystal capacitor Clc in order to suppress reduction in charges(voltage) retained in the liquid crystal capacitor Clc due to a leakcurrent of the liquid crystal layer. The storage capacitor Cs is aso-called parallel-electrode condenser (capacitor) that uses as oneelectrode a corresponding scanning line (gate bus line) or a Cs bus lineformed from the same conductive layer as that of the scanning line, andalso uses as the other electrode a conductive layer (typically, ITOlayer) forming the picture-element electrode. A dielectric between theseelectrodes is formed from, e.g., a TaO_(x) layer and a SiN_(x) layerformed thereon, like a gate insulating film of the TFT. The capacitanceof the storage capacitor Cs indicates an electrostatic capacitance ofthe storage capacitor Cs. For simplicity, “Cs” herein indicates both thestorage capacitor itself and the electrostatic capacitance thereof.

[0168] The capacitance of the liquid crystal capacitor Clc indicates anelectrostatic capacitance of the liquid crystal capacitor Clc. Forsimplicity, “Clc” herein indicates both the liquid crystal capacitoritself and the electrostatic capacitance thereof. Note that the liquidcrystal capacitor Clc is a capacitor using the liquid crystal layer as adielectric layer, and the dielectric constant of the liquid crystallayer changes as the orientation state of the liquid crystal layerchanges according to the applied voltage. Accordingly, the capacitanceratio of the storage capacitor Cs to the liquid crystal capacitor Clcchanges according to the applied voltage. Thus, the aforementionedrelation of the capacitance ratio of the storage capacitor Cs to theliquid crystal capacitor Clc, i.e., Cs/Clc.1, is herein based on thecapacitance of the liquid crystal capacitor Clc (the maximum capacitancein the actual display) at the time when the highest gray-scale voltage(e.g., 7 V) is applied to the picture-element capacitor Cpix.

[0169] Hereinafter, a signal that provides image information to bedisplayed on the LCD is ref erred to as an input image signal S, and avoltage that is applied to the picture-element capacitor Cpix accordingto each input image signal S is referred to as a gray-level voltage Vg.

[0170] It is known that the TFT-type LCD exhibit step responsecharacteristics as its response characteristics. FIG. 13 schematicallyshows the step response characteristics of the optical characteristics(transmittance) of the TFT-type LCD. In FIG. 13, the ordinate indicatesa transmittance, but this can be replaced with a charging voltage of thepicture-element capacitor Cpix. The principles of the step responsecharacteristics of the transmittance (or charging voltage) will now bedescribed with reference to FIG. 13.

[0171] In the TFT-type LCD, the amount of charges (Q) stored in a singlepicture-element capacitor Cpix is determined from the voltage (V)applied to the picture-element capacitor Cpix during the ON state of thecorresponding TFT and the capacitance of the picture-element capacitorCpix (C=Clc+Cs) at that time. Herein, the ON state of the TFT is aperiod during which a scanning voltage is applied to the gate electrodethereof, and this period is also referred to as a horizontal scanningperiod. Moreover, the voltage (V) applied to the picture-elementcapacitor Cpix corresponds to the potential difference between thecorresponding picture-element electrode and the counter electrode. Thisrelation is given by the expression: Q=CV. In other words, when the TFTis turned ON, the corresponding picture-element capacitor Cpix ischarged until the amount of charges (Q) determined by Q=CV is storedtherein. If the picture-element capacitor Cpix retains 100% of thevoltage (i.e., if there is no leak current), the charges (Q) areretained until the TFT is again turned ON in the following field (orframe; hereinafter, a single field is used).

[0172] In the period during which the picture-element capacitor Cpixretains the charges loaded therein (this period corresponds to a singlefield), the voltage (V) of the picture-element capacitor Cpix decreasesgradually. This is because the liquid crystal molecules of Δε>0 that areoriented in parallel with the electrode plane of the pair of opposingelectrodes are raised in the direction normal to the electrode planeaccording to the applied voltage (i.e., the liquid crystal molecules areoriented in parallel with the electric field). According to this changein orientation of the liquid crystal molecules, the dielectric constantof the liquid crystal layer is increased, whereby the capacitance of theliquid crystal capacitor Clc is increased. In other words, thecapacitance of the picture-element capacitor Cpix is increased. As thecapacitance (C) of the picture-element capacitor Cpix is increased, thevoltage (V) on the picture-element capacitor Cpix is reduced accordingto the relation: Q=CV. Thus, the voltage retained in the picture-elementcapacitor Cpix is reduced during a single field, whereby thetransmittance (or charging voltage) changes stepwise on a field-by-fieldbasis (step response), as shown in FIG. 13.

[0173] Note that this step response does not occur in a so-called staticdriving method in which a voltage is continuously applied to thepicture-element capacitor cpix over a single field. Thus, the TFT-typeLCD including a step-responding liquid crystal panel has a lowerresponse speed than that of the statically driven LCD in which a voltageis continuously applied to the liquid crystal layer. As a result, thedegree of residual image is increased, degrading the moving picturedisplay quality.

[0174] In the LCD according to the second aspect of the presentinvention, the capacitance ratio of the storage capacitor Cs to theliquid crystal capacitor Clc satisfies the relation: Cs/Clc.1.Therefore, even if the capacitance of the liquid crystal capacitor Clcis increased according to a change in orientation of the liquid crystalmolecules, a change in capacitance of the picture-element capacitor Cpixis suppressed. Accordingly, the aforementioned step response of thetransmittance (or charging voltage) is suppressed. Moreover, providedthat the capacitance ratio of the storage capacitor Cs to the liquidcrystal capacitor Clc satisfies the relation: Cs/Clc.1, thepicture-element capacitor Cpix can retain 90% or more of the chargingvoltage corresponding to the input image signal S over a single field.As a result, the liquid crystal panel can attain 90% or more of aprescribed transmittance corresponding to the input image signal Swithin a single field. In order to increase the capacitance of thestorage capacitor Cs, it is only necessary to increase the area of thestorage capacitor Cs, or reduce the thickness of the dielectric layer,or form the dielectric layer from a material having a larger dielectricconstant.

[0175] Assuming that the input image signal S (60 Hz) is changed fromthe lowest gray-level voltage (Vv0) to the highest gray-level voltage(e.g., Vv63) in the NW mode LCD, a change in transmittance with timewill be described with reference to FIG. 14. The abscissa of FIG. 14 isscaled every field, i.e., every 16.7 msec, from the point where theinput image signal S is shifted. Three curves in the figure show achange in transmittance with time for the liquid crystal panels that aredifferent in the capacitance ratio of the storage capacitor Cs to theliquid crystal capacitor Clc (Cs/Clc) and in viscosity of the liquidcrystal material. In FIG. 14, the transmittance after one fieldcorresponds to about 95% of the target transmittance in curve L1, about90% in curve L2, and about 60% in curve L3.

[0176] As shown in FIG. 14, the relation between the transmittance afterone field (after 16.7 msec) and the number of fields required for thetransmittance to reach the target value shows that, in the case wherethe transmittance after one field corresponds to approximately 90% ormore of the target value, the transmittance reaches the target valuewithin two fields (within 33.4 msec), as shown by curves LI and L2. Incontrast, in the case where the transmittance after one fieldcorresponds to less than 90% of the target value (in the case of theconventional LCD), it takes more than two fields for the transmittanceto reach the target value, as shown by curve L3 of FIG. 14.

[0177] As a result of comparison of the moving picture displaycharacteristics between the LCD requiring more than two fields for thetransmittance to reach the target value and the LCD whose transmittancereaches the target value within two fields, the residual image wasobviously reduced more in the latter LCD than in the former LCD.

[0178]FIG. 15 shows a change in transmittance in the NW mode LCDs havingvarious Cs/Clc values in the case where the input image signals S(gray-level voltages Vg) of the previous and current fields aredifferent from each other. The transmittance ratio of the ordinateindicates the ratio of a transmittance after one field to a steady-statetransmittance of the gray-level voltage Vg corresponding to the inputimage signal S of the current field. More specifically, in the casewhere a prescribed transmittance of the current field is reached withinone field, the transmittance ratio of the ordinate is 1. In the legend,the numerical values on the left side indicate a gray-level voltage ofthe previous field (e.g., 48 indicates the gray-level voltage Vv48), andthe numerical values on the right side indicate a gray-level voltage ofthe current field. In the case of the 64-gray-scale display, Vv0 is thelowest gray-level voltage, and Vv63 is the highest gray-level voltage(corresponding to the highest limit signal). It can be seen from FIG. 15that, with the value Cs/Clc being set to 1 or more, the transmittanceafter one field corresponds to 90% or more of a steady-statetransmittance (the transmittance ratio is 0.9 or more) when the highestgray-level voltage Vv63 is applied. In other words, with the valueCs/Clc being set to 1 or more, the picture-element capacitor Cpixretains 90% or more of the charging voltage over one field when thehighest gray-level voltage Vv63 is applied.

[0179] (Overshoot Driving)

[0180] As described above, setting the value Cs/Clc to 1 or more allowsthe transmittance to reach 90% or more of a steady-state transmittanceafter one field when the highest gray-level voltage Vv63 is applied.However, when a gray-level voltage (intermediate-gray-level voltage)lower than the highest gray-level voltage Vv63 is applied for each graylevel, the response speed is improved, but still is not enough.Therefore, even if the value Cs/Clc is set to 1 or more, thetransmittance ratio after one field does not reach 0.9.

[0181] Such a response speed in the intermediate-gray-scale displaystate can be improved by overshoot driving described in the firstembodiment. More specifically, according to combination of therespective input image signals S of the previous field and the currentfield, a predetermined driving voltage overshooting the gray-levelvoltage Vg corresponding to the input image signal S of the currentfield is supplied to the liquid crystal panel.

[0182] As described in the first embodiment, comparison of the inputimage signal S for detecting the overshoot voltage is made between therespective input image signals S of the previous and current fields forevery picture element. Even in the interlace driving in which imageinformation corresponding to a single frame is divided into a pluralityof fields, the input image signal S of a picture element of interest inthe previous frame and the input image signals S of the upper and lowerlines are used as supplementary signals, so that the signalscorresponding to all the picture elements are applied within a singlevertical period. Thus, the input image signals S of the previous andcurrent fields are compared with each other.

[0183] The overshoot voltage may either be another gray-level voltage Vghaving a prescribed overshoot amount with respect to a prescribedgray-level voltage Vg, or a dedicated overshoot-driving voltage that isprepared in advance for the overshoot driving. In order to improve theresponse speed of the intermediate-gray-scale display state, anovershoot driving voltage that is set based on the gray-level voltage Vgis used. A dedicated overshoot-driving voltage may be used for furtherimprovement in response speed.

[0184] (Circuit for Conducting Overshoot Driving)

[0185] The driving circuit in the LCD of the present embodiment has thesame structure as that of the driving circuit 10 described in the firstembodiment in connection with FIG. 14. Therefore, description thereof isomitted.

[0186] Hereinafter, the input/output signal of each circuit will bedescribed with reference to FIG. 4. In the following description, it isassumed that a voltage used for overshoot driving is preset to agray-level voltage Vg that is higher than the gray-level voltage Vgcorresponding to the input image signal S.

[0187] First, the image storage circuit 11 retains the input imagesignal S corresponding to one field before the input image signal S ofthe current field. The combination detection circuit 12 detects, forevery picture element, a combination of the input image signal S of thecurrent field and the input image signal S of the previous fieldretained in the image storage circuit 11. For convenience, thecombination of the input image signals S (gray-level data) detected bythe combination detection circuit 12 is indicated by a combination ofthe corresponding gray-level voltages. For example, in the NW mode, thecombination of the input image signal S63 of the previous field and theinput image signal S35 of the current field is indicated by acombination of the corresponding gray-level voltages (Vv0, Vv28).

[0188] The overshoot voltage detection circuit 13 detects a gray-levelvoltage Vv44 that is predetermined for the combination (Vv0, Vv28)detected by the combination detection circuit 12, and supplies thegray-level voltage Vv44 to the polarity inversion circuit 14 as adriving voltage. This operation corresponds to conversion of thegray-level voltage Vv28 corresponding to the input image signal S of thecurrent field to the gray-level voltage Vv44. For example, the processof detecting the gray-level voltage Vv44 as a predetermined overshootvoltage corresponding to the combination (Vv0, Vv28) detected by thecombination detection circuit 12 may be conducted either by a lookuptable method or by performing a predetermined operation.

[0189] Finally, the polarity inversion circuit 14 converts thegray-level voltage Vv44 to an AC signal for supply to the liquid crystalpanel 15.

[0190] A specific method for setting the overshoot gray-level voltage Vg(driving voltage) for the input image signal S of the current field willbe described. In the following description, it is assumed that thegray-level voltage corresponding to the input image signal S of theprevious field is Vv0, and the gray-level voltage corresponding to theinput image signal S of the current field is Vv28, and that theovershoot gray-level voltage Vv44 (which overshoots Vv28) is used as adriving voltage.

[0191]FIG. 16 shows a change in transmittance with time according to achange in gray-level voltage (input image signal). The solid line showsthe case where the gray-level voltage Vv28 of the current field issupplied in the state where the transmittance is stable at asteady-state transmittance of the gray-level voltage Vv0 of the previousfield, and the gray-level voltage Vv28 is continuously supplied in thefollowing fields. A single field corresponds to 16.7 msec. The dashedline in FIG. 16 shows the case where the gray-level voltage Vv44 of thecurrent field is supplied in the state where the transmittance is stableat a steady-state transmittance of the gray-level voltage Vv0 of theprevious field, and the gray-level voltage Vv44 is continuously suppliedin the following fields.

[0192] It can be seen from FIG. 16 that it takes about three fields fromapplication of the gray-level voltage Vv28 until the transmittancebecome stable. In other words, it takes about three fields for thetransmittance to reach a steady state transmittance of the gray-levelvoltage Vv28. On the other hand, in the case of the gray-level voltageVv44, the transmittance reaches the steady state transmittance of thegray-level voltage Vv28 after about one field, and then goes toward asteady state transmittance of the gray-level voltage Vv44.

[0193] As can be seen from this, in order to change (update) thetransmittance of the liquid crystal panel from the steady-statetransmittance of Vv0 to that of Vv28 within a single field, thegray-level voltage Vv44 need only be supplied instead of Vv28. Thus, forevery combination of the input image signals S (combination of theprevious and current fields), an overshoot voltage is determined so thatthe transmittance reaches within a single field a steady statetransmittance (desired transmittance) of the gray-level voltage Vgcorresponding to the input image signal S of the current field.

[0194] Hereinafter, a method for conducting overshoot driving for everygray-level voltage will be described. In particular, a method forsetting an overshoot voltage for the highest gray-level voltage (Vv63)and the lowest gray-level voltage (Vv0) will be described. Herein, thedescription will be exemplarily given for the case of the highestgray-level voltage.

[0195] First, voltages of 128 gray levels (Vv′0 to Vv′127) are preparedin advance for gray-level voltages of 64 gray levels (Vv0 to Vv63). Forexample, the voltages Vv′32 to Vv′95 (64 gray levels) are assigned tothe voltages Vv0 to Vv63 (64 gray levels). The voltages Vv′0 to Vv′31are used as a lower dedicated overshoot-driving voltage, and thevoltages Vv′96 to Vv′127 are used as a higher dedicatedovershoot-driving voltage.

[0196] For example, it is now assumed that the gray-level voltagecorresponding to the input image signal S is shifted from Vv44 to Vv63after one field. These gray-level voltages Vv44 and Vv63 are input tothe image storage circuit 11 (see FIG. 4) as digital signalsrespectively corresponding to Vv′76 and Vv′95 by a circuit for assigningthe gray-level voltages of 128 gray levels (i.e., a circuit forconverting a 6-bit digital signal to a 7-bit digital signal). Thecombination detection circuit 12 detects the combination (Vv′76, Vv′95).Then, the overshoot voltage detection circuit 13 detects the voltageVv′100 that is predetermined so as to attain a steady-statetransmittance of Vv′95 within one field, and then outputs the voltageVv′100 to the polarity inversion circuit 14 as a driving voltage. Thisdriving voltage Vv′100 is then converted into an AC signal in thepolarity inversion circuit 14 for supply to the liquid crystal panel 15.In the case of the lowest gray-level voltage (Vv0) as well, a drivingvoltage lower than the lowest gray-level voltage (Vv0) can be similarlysupplied to the liquid crystal panel 15.

[0197] Thus, the voltages of 128 gray levels (including dedicatedovershoot-driving voltage of 64 gray levels) are prepared in advance forthe gray-level voltages of 64 gray levels. This makes it possible to usea voltage higher than the highest gray-level voltage (Vv63 of the 64gray levels) and a voltage lower than the lowest gray-level voltage(Vv0) as an overshoot voltage. In this case, however, improvement inwithstand voltage of the driver and/or extension of the controller arerequired.

[0198] As described above, by conducting the overshoot driving with thecapacitance ratio of the storage capacitor Cs to the liquid crystalcapacitor Clc (Cs/Clc) being set to 1 or more, an increased responsespeed is implemented for every gray level. Overshoot driving using agray-level voltage in the range of Vv0 to Vv63 is effective even when avoltage lower than Vv0 and/or a voltage higher than Vv63 cannot beapplied to the liquid crystal panel in view of the withstand voltage ofthe driver (driving circuit, and typically, driver IC) and extension ofthe controller).

[0199] Although the optical response characteristics (corresponding tocharging characteristics) have been described for the case where thegray-level voltage is changed from a lower gray-level voltage to ahigher gray-level voltage (i.e., rise of the response), the presentinvention is also effective in improving the optical responsecharacteristics (corresponding to discharging characteristics) in thecase where the gray-level voltage is changed from a higher gray-levelvoltage to a lower gray-level voltage (fall of the response). Since theliquid crystal response upon discharging is relatively slow as comparedto that upon charging, the effect of overshoot driving is rather likelyto be observed as improvement in fall response characteristics.

[0200] A specific example of the method for setting an overshoot voltageis shown in Table 1. Table 1 shows the case where the capacitance ratioof the storage capacitor Cs to the liquid crystal capacitor Clc is 1 ormore. For comparison, Table 2 shows the case where the capacitance ratioof the storage capacitor Cs to the liquid crystal capacitor Clc is lessthan 1.

[0201] In each table, the numerical values in the right column indicategray-level data regarding a gray-level voltage corresponding to theinput image signal S of the previous field (the field immediatelypreceding the field to be displayed) (e.g., 255 for the gray-levelvoltage Vv255). The numerical values in the bottom row indicategray-level data regarding a gray-level voltage corresponding to theinput image signal S of the current field (the field to be displayed).The numerical values in each column of Tables 1 and 2 indicate theovershoot amount required to attain within a single field a steady-statetransmittance of the gray-level voltage corresponding to the input imagesignal S of the current field. These numerical values indicate theovershoot amount as the difference in gray level. For example, thenumerical value “−39” in the ninth row, third column of Table 1indicates that the gray-level voltage Vv255 (64−39=25) must be suppliedas a driving voltage in order to provide the display corresponding toVv64 in the current field after providing the display corresponding toVv255 in the previous field. As can be seen from the tables, it ispreferable to adjust the overshoot amount according to the gray-leveldata of the previous field, even if the gray-level data of the currentfield is the same. Moreover, comparison between Tables 1 and 2 showsthat, in the case where the capacitance ratio of the storage capacitorCs to the liquid crystal capacitor Clc is less than 1 (Table 2), alarger overshoot amount is required as the gray-level data of thecurrent field is greater. In other words, it is appreciated that theresponse characteristics in the high-band (the region where thegray-level voltage is high) can be improved by setting the capacitanceratio of the storage capacitor Cs to the liquid crystal capacitor Clc to1 or more, as described above.

[0202] In Tables 1 and 2, the numerical values having the symbol “*”attached thereto indicates that, with that overshoot amount, asteady-state transmittance of the gray-level voltage corresponding tothe input image signal S of the current field is not reached within onefield. In other words, a dedicated overshoot-driving voltage must beprovided separately. TABLE 1 Cs/Clc · 1 0 7 7 8 21 23 63* 31* 0 0 0 0 77 20 22 56 31* 0 32 0 −4 0 7 16 18 54 31* 0 64 0 −5 −4 0 14 17 51 31* 096 0 −9 −5 −4 0 11 45 31* 0 128 0 −9 −8 −8 −7 0 38 31* 0 160 0 −19 −20−14 −17 −14 0 25 0 192 0 −25 −26 −21 −25 −26 −14 0 0 224 0 −32* −39 −37−37 −48 −36 −42 0 225 0 −32 64 96 128 160 192 224 255

[0203] TABLE 2 Cs/Clc < 1 0 8 31 55 56 55 50 27 0 0 0 0 25 55 56 55 4827 0 32 0 −16 0 18 36 40 44 27 0 64 0 −23 −7 0 26 32 40 27 0 96 0 −27−11 −14 0 19 38 26 0 128 0 −31 −14 −16 −19 0 24 25 0 160 0 −31 −20 −30−33 −19 0 24 0 192 0 −32* −33 −38 −41 −48 −31 0 0 224 0 −32* −64* −66−89 −115 −36 −120 0 255 0 32 64 96 128 160 192 224 255

[0204] (Liquid Crystal Material)

[0205] A liquid crystal material having a large value ε// and alsohaving a value Δε that is small to such a degree that does not degradethe response capability is preferred for use in the LCD according to thesecond aspect of the present invention. The reason for this will bedescribed below.

[0206] In order to reduce the step response resulting from an increasein capacitance of the picture-element capacitor cpix (voltage drop)according to a change in orientation of the liquid crystal molecules, itis preferable that the difference between the capacitance in verticalorientation of the liquid crystal molecules and the capacitance inparallel orientation thereof is small. In other words, for a liquidcrystal material having a positive dielectric anisotropy (Δε>0), it ispreferable that (Cs +Clc⊥)/(Cs +Clc//)=1−Δε(S/d)/(Cs +Clc//) is large.Clc⊥ and Clc// indicate the capacitance of the liquid crystal capacitorClc in vertical orientation of the liquid crystal molecules and inparallel orientation thereof, respectively. Moreover, Δε=ε//−ε⊥),Clc⊥=ε₀·ε⊥(S/d), and Clc//=ε₀·ε//(S/d). S indicates the area of apicture element (typically, picture-element electrode) of the liquidcrystal capacitor Clc, and d indicates the thickness of the liquidcrystal layer.

[0207] Thus, it is preferable that Δε is small. However, if Δε is small,the response capability of the liquid crystal molecules to the electricfield is degraded. Therefore, it is preferable that Δε is not reduced asmuch as possible and that ε// is large. In general, however, as ε// isincreased, the viscosity of the liquid crystal material is increased,degrading the response capability of the liquid crystal molecules to theelectric field. Accordingly, it is preferable that the viscosity of theliquid crystal material is as low as possible.

[0208] Although the present embodiment has been described for the NWmode LCD, the LCD according to the second aspect of the presentinvention is also applicable to the NB mode LCD.

[0209] (Display Mode)

[0210] The LCD according to the second aspect of the present inventionis applicable to various LCDs. The response characteristics of theliquid crystal panel depend on the response speed of the liquid crystallayer (liquid crystal material, orientation mode and the like).Accordingly, by using a liquid crystal layer having a high responsespeed, an LCD having rapid response characteristics and excellentviewing-angle characteristics can be obtained. Moreover, by applying thepresent invention to such an LCD, the residual image can be moreeffectively reduced, whereby an LCD having excellent viewing-anglecharacteristics and high image quality can be obtained.

[0211] For example, the present invention can be applied to the ECB(Electrically Controlled Birefringence) mode, transmission-type liquidcrystal panel 20 using a parallel-orientation (homogeneous-orientation)liquid crystal layer, which is described in the first embodiment inconnection with FIG. 7. Note that, since the structure of thetransmission-type liquid crystal panel 20 is the same as that describedin the first embodiment, description thereof is herein omitted.

[0212] In the liquid crystal panel 20 having the parallel-orientationliquid crystal layer, the retardation d·Δn of the liquid crystal layer27 alone, i.e., the retardation except the phase compensators 23 and 24,is preferably in the range of about 270 nm to about 340 nm. With thethickness of the liquid crystal layer 27 being 4.5 μm, Δn=0.06 to 0.075,whereby a liquid crystal material having a smaller refractive indexanisotropy Δn than the typical value Δn=about 0.08 of the TN mode liquidcrystal material can be used. For example, the liquid crystal materialof the liquid crystal layer 27 has a refractive index anisotropy (Δn) of0.06, and the thickness of the liquid crystal layer 27 is adjusted to4.5 μm.

[0213] In general, the viscosity of the liquid crystal materialdecreases with decrease in Δn. This is also effective in reduction ofthe response time of the liquid crystal layer. On the contrary, in thecase of using the liquid crystal material of Δn=about 0.08 as in the TNmode liquid crystal panel, the thickness of the liquid crystal layer 27can further be reduced. As the thickness of the liquid crystal layer 27is reduced, the response time is reduced approximately in proportion tothe square of the reduction in thickness. Accordingly, the use of thehomogeneous-orientation liquid crystal layer achieves significanteffects in improving not only the viewing angle characteristics but alsothe moving picture display quality.

[0214] Moreover, an optical element for diffusing the light transmittedin or near the direction normal to the display plane (i.e., the displaylight) in the upward and downward directions with respect to the line ofsight of the viewer, that is, an optical element having the lens effectonly in a one-dimensional direction (e.g., BEF made by Sumitomo 3M Ltd.)is provided on the display plane of the liquid crystal panel 20. Thus,the liquid crystal panel having nearly constant display qualityregardless of the viewing angle, and thus having an extremely wideviewing angle can be obtained.

[0215]FIG. 17 schematically shows an ECB (Electrically ControlledBirefringence) mode liquid crystal panel 100 using aparallel-orientation (homogeneous-orientation) liquid crystal layer. TheECB mode is known as a liquid crystal mode of the NB mode having a fastresponse speed and excellent viewing-angle characteristics.

[0216] The liquid crystal panel 100 includes a liquid crystal layer 101,a pair of electrodes 100 a and 100 b for applying a voltage to theliquid crystal layer 101, a pair of phase plates (of course, phasecompensation films may be used) 102 and 103 provided on both sides ofthe liquid crystal layer 101, phase plates 104, 105 and phase plates110, 111 provided on the respective outer surfaces of the phase plates102 and 103, and a pair of polarizing plates 108 and 109 interposingthese elements therebetween and arranged in the crossed nicols state.Note that the phase plates 104, 105 and the phase plates 110, 111 mayeither be omitted, or one or a plurality of phase plates may be providedin any combination.

[0217] The arrow in each phase plate in FIG. 17 indicates an axis of itsindex ellipsoid (every index ellipsoid has a positive, uniaxialproperty) that has the maximum refractive index (i.e., a slow axis). Thearrow in each polarizing plate 108, 109 indicates an polarization axisthereof (polarization axis=transmission axis, and polarization axisabsorption axis).

[0218]FIG. 17 shows orientation of the liquid crystal molecules (shownby ellipses in FIG. 17) within a single picture-element region in theliquid crystal layer 101 in the state where a voltage is not applied. Anematic liquid crystal material having a positive dielectric anisotropyis used as the liquid crystal material. When a voltage is not applied,the liquid crystal molecules are oriented approximately in parallel withthe surface of a pair of substrates (not shown). The electrodes 100 aand 100 b are respectively formed on the pair of substrates so as toface the liquid crystal layer 101 and to interpose the liquid crystallayer 101 therebetween. In response to application of the voltage to theelectrodes 100 a and 100 b, an electric field is produced in the liquidcrystal layer 101 in the direction approximately perpendicular to thesubstrate surface. As shown in FIG. 17, the liquid crystal layer 101 hasfirst and second domains 101 a and 101 b within each picture elementregion. The first and second domains 101 a and 101 b have differentorientation states from each other. In the example of FIG. 17, thedirector of the liquid crystal molecules in the first domain 101 a isoriented in an azimuth direction that is different by 180° from that ofthe director of the liquid crystal molecules in the second domain 101 b.

[0219] The orientation of the liquid crystal molecules is controlledsuch that the liquid crystal molecules within the first domain 101 a areraised clockwise as well as the liquid crystal molecules within thesecond domain 101 b are raised counterclockwise in response toapplication of a voltage between the electrodes 100 a and 101 b. Inother words, the orientation of the liquid crystal molecules iscontrolled such that the liquid crystal molecules in the first andsecond domains 101 a and 101 b are raised in the opposite directions.Such orientation of the directors of the liquid crystal molecules can beimplemented by the known alignment control technology using an alignmentfilm. In the case where a plurality of first and second domains havingthe orientation directions of the respective directors different fromeach other by 180° are formed within a single picture-element region,the display characteristics can be averaged by smaller units. Therefore,further uniform viewing-angle characteristics can be obtained.

[0220] Each of the phase plates 102 and 103 typically has a positive,uniaxial refractive index anisotropy, and its slow axis (the arrow inFIG. 17) extends orthogonally to a slow axis (not shown) of the liquidcrystal layer 101 in the state where a voltage is not applied.Accordingly, light leakage (degradation in black display level) can besuppressed which results from the refractive index anisotropy of theliquid crystal molecules in the state where a voltage is not applied (inthe black display state).

[0221] Each of the phase plates 104 and 105 typically has a positive,uniaxial refractive index anisotropy, and its slow axis (the arrow inFIG. 17) extends perpendicularly to the substrate surface (i.e.,perpendicularly to the respective slow axes of the liquid crystal layer101, and phase plates 102 and 103), so as to compensate for a change intransmittance according to a change in viewing angle. Accordingly, withthe use of the phase plates 104 and 105, the display having moreexcellent viewing-angle characteristics can be provided. Both of thephase plates 104 and 105 may be omitted. Alternatively, only one of thephase plates 104 and 105 may be used.

[0222] Each of the phase plates 110 and 111 typically has a positive,uniaxial refractive index anisotropy, and its slow axis (the arrow inFIG. 17) extends orthogonally to the polarization axis of thecorresponding polarizing plates 108, 109 (i.e., makes an angle of 45°with the respective slow axes of the liquid crystal layer 101, and phaseplates 102 and 103), so as to adjust the rotation of the polarizationaxis of elliptic polarization. Accordingly, with the use of the phaseplates 110 and 111, the display having more excellent viewing-anglecharacteristics can be provided. Both of the phase plates 110 and 111may be omitted. Alternatively, only one of the phase plates 110 and 111may be used. The phase plates 102, 103, 104, 105, 110 and 111 do notnecessarily have a uniaxial refractive index anisotropy, but may have apositive, biaxial refractive index anisotropy.

[0223] (Embodiment 3)

[0224] An LCD of the third embodiment is a TFT-type LCD as shown in FIG.12. More specifically, the LCD of the third embodiment is a NW modedisplay device including the liquid crystal panel 20 shown in FIG. 7 andthe driving circuit 10 shown in FIG. 4. This LCD will be described withreference to FIGS. 4, 7 and 12.

[0225] The TFT substrate 21 and CF substrate 22 forming the TFT-typeliquid crystal panel are made according to a known method. Thecapacitance of a single storage capacitor Cs of the TFT substrate 21 is,e.g., 0.200 pF. An alignment film (which is formed from, e.g., polyimideor polyvinyl alcohol) is formed on each of the respective surfaces ofthe substrates 21 and 22 that face the liquid crystal layer 27. Then,the surface of each alignment film is rubbed in one direction.

[0226] The TFT substrate 21 and CF substrate 22 thus obtained arelaminated with each other such that their respective rubbing directionsare in anti-parallel with each other. Then, a nematic liquid crystalmaterial of Δε>0 is introduced therebetween, whereby the liquid crystalcell 20 a is obtained. The capacitance of a single liquid crystalcapacitor Clc of the liquid crystal cell 20 a is, e.g., 0.191 pF (whenthe highest gray-level voltage (7 V) is applied).

[0227] The phase plates 23 and 24 are respectively laminated to theouter surfaces of the TFT substrate 21 and CF substrate 22. The phaseplates 23 and 24 are arranged such that the inclination direction of therespective index ellipsoids (counterclockwise in FIG. 7) is opposite tothe pre-tilt direction of the liquid crystal molecules (clockwise inFIG. 7). Moreover, the pair of polarizers 25 and 26 are respectivelylaminated on the outer surfaces of the phase plates 23 and 24 so thatthe respective absorption axes of the polarizers extend orthogonally toeach other and also make an angle of 45° with the rubbing direction.Thus, the liquid crystal panel 20 is obtained.

[0228] As described in the first embodiment in connection with FIG. 4,the driving circuit 10 receives an external input image signal S, andsupplies a corresponding driving voltage to the liquid crystal panel 15.The driving circuit 10 includes the image storage circuit 11,combination detection circuit 12, overshoot voltage detection circuit13, and polarity inversion circuit 14.

[0229] The image storage circuit 11 retains at least one field image ofthe input image signal S. The combination detection circuit 12 comparesthe input image signal S of the current field with the input imagesignal S of the previous field retained in the image storage circuit 11,and outputs a signal indicating that combination to the overshootvoltage detection circuit 13. The overshoot voltage detection circuit 13detects a driving voltage corresponding to the combination detected bythe combination detection circuit 12, from the gray-level voltage Vg andthe dedicated overshoot-driving voltage.

[0230] The polarity inversion circuit 14 converts the driving voltagedetected by the overshoot voltage detection circuit 13 into an AC signalfor supply to the liquid crystal panel (display section) 15. Herein, theovershoot voltage is conducted also to the highest and lowest gray-levelvoltages.

[0231]FIG. 18A shows respective response characteristics of the LCD ofthe present embodiment and a conventional LCD. The input image signal Sis a signal at 60 Hz for one field, and the gray level changes rapidlyin the third field from the gray level of the second field. As shown inFIG. 18B, in response to the change in gray level in the third field,the driving circuit 10 of the present embodiment supplies as a drivingvoltage an overshoot voltage to the liquid crystal panel 15 in the thirdfield. More specifically, this overshoot voltage is a voltageovershooting (by the overshoot amount OS in the figure) the gray-levelvoltage corresponding to the input image signal S of the third field(this gray-level voltage is applied in the four and the followingfields). From the third field, the input image signal S does not haveany change in gray level. Therefore, the driving circuit 10 supplies asa driving voltage the gray-level voltage corresponding to the inputimage signal S to the liquid crystal panel 15 without overshooting thegray-level voltage.

[0232] As is apparent, the overshoot gray-level voltage (having its highband being enhanced) is supplied to the liquid crystal panel 15 in thethird field, whereby the response characteristics are significantlyimproved over the conventional LCD (dashed line in the figure) in whicha non-overshoot voltage gray-level voltage is applied.

[0233] (Embodiment 4)

[0234] An LCD of the fourth embodiment is a TFT-type LCD as shown inFIG. 12. More specifically, the LCD of the fourth embodiment is a NBmode display device including the liquid crystal panel 100 shown in FIG.17 and the driving circuit 10 shown in FIG. 4. This LCD will bedescribed with reference to FIGS. 4, 12 and 17.

[0235] The TFT substrate 100 b and CF substrate 100 a forming theTFT-type liquid crystal panel 100 are made according to a known method.The capacitance of a single storage capacitor Cs of the TFT substrate100 b is, e.g., 0.200 pF.

[0236] An alignment film is formed on each of the respective surfaces ofthe substrates 100 a and 100 b that face the liquid crystal layer 101.The surface of each alignment film is divided into two regions A and Bin every picture element, and ultraviolet light (UV radiation) isradiated to the regions A and B. In the region A, the UV light isradiated to the alignment film on the CF substrate 100 a. In the regionB, the UV light is radiated to the alignment film on the TFT substrate100 b. Then, the surface of each alignment film is rubbed in a singledirection. The TFT substrate 100 b and CF substrate 100 a are laminatedwith each other such that their respective rubbing directions are inparallel with each other. Then, a nematic liquid crystal material ofΔε>0 is introduced therebetween, whereby a liquid crystal cell isobtained. The capacitance of a single liquid crystal capacitor Clc ofthe liquid crystal cell thus obtained is, e.g., 0.191 pF (when thehighest gray-level voltage (7 V) is applied).

[0237] The orientation state of the liquid crystal molecules in thisliquid crystal cell will be described with reference to FIGS. 19A to19C. FIG. 19A shows that the two regions A and B within a single pictureelement 201 have the same rubbing direction 202, 203. As shown in FIG.19B, if the above UV radiation is not conducted, liquid crystalmolecules 206 located approximately in an intermediate layer of theliquid crystal layer are oriented approximately in parallel with thesubstrate surface when a voltage is not applied. When a voltage isapplied to the liquid crystal layer, the liquid crystal molecules 206located in the intermediate layer are raised in the direction shown bythe arrow 207 or 208 with the same probability.

[0238] However, since the alignment films 205 and 204 have beensubjected to the UV radiation in the regions A and B, respectively, thepre-tilt angle is reduced on the UV-radiated alignment films. As aresult, as shown in FIG. 19C, the liquid crystal molecules 206 locatedapproximately in the intermediate layer of the liquid crystal layer inthe region A are rotated in the direction shown by the arrow 207,whereas the liquid crystal molecules 206 located approximately in theintermediate layer of the liquid crystal layer in the region B arerotated in the direction shown by the arrow 208. In other words, thealignment is controlled such that the pre-tilt direction of the liquidcrystal molecules 206 located near the intermediate layer of the liquidcrystal layer is different by 180° between the regions A and B. In theliquid crystal layer having such an orientation state, the two regions Aand B compensate for the viewing-angle dependency with each other,resulting in excellent viewing-angle characteristics. Note that theliquid crystal layer having the aforementioned orientation is preferred.However, the viewing-angle characteristics can be improved by using aliquid crystal layer that has two or more regions having differentorientation states of the liquid crystal molecules.

[0239] The phase plates and the polarizing plates are laminated onto theresultant liquid crystal cell as shown in FIG. 17, whereby the liquidcrystal panel 100 is obtained.

[0240] Each region has the following alignment parameters. TABLE 3 Ratioof Occupied area within picture Twist Alignment Region elementRetardation angle direction A 50% 240 nm 0 deg  0 deg B 50% 240 nm 0 deg180 deg

[0241] The polarizing plates 108 and 109 have the following parameters.Note that the angle of the transmission axis of each polarizing plate108, 109 is an angle with respect to the orientation direction of theliquid crystal molecules. TABLE 4 Polarizing plate No. Angle oftransmission axis 108 45 deg 109 45 deg

[0242] The phase plates 102 to 105, 110 and 111 have the followingparameters. In Table 5, na, nb and nc are three principal refractiveindices of the index ellipsoid of the phase plate; d is the thickness ofthe phase plate; d·(na−nb) is a retardation within a plane that is inparallel with the display plane of the liquid crystal panel 100; andd·(na−nc) is a retardation in the thickness direction. The angle ofna-axis is an angle with respect to the orientation direction of theliquid crystal molecules. TABLE 5 Phase plate Angle of No. d.(na-nb)D.(na-nc) na-axis 102 120 nm    0 nm 90 deg 103 120 nm    0 nm 90 deg104  0 nm −120 nm 90 deg 105  0 nm −120 nm 90 deg 110  25 nm    0 nm −45deg   111  25 nm    0 nm 45 deg

[0243] The liquid crystal panel 100 has the regions A and B in everypicture element, which have different orientation directions of theliquid crystal molecules. Moreover, the phase plates compensate for theviewing-angle characteristics. Accordingly, the liquid crystal panel 100has wide viewing-angle characteristics.

[0244] Since the driving circuit 10 is the same as that of the thirdembodiment, description thereof is herein omitted.

[0245]FIG. 20 shows response characteristics of the LCD of the presentembodiment. As in the third embodiment, the input image signal S is asignal at 60 Hz for one field, and the gray level changes rapidly in thethird field from the gray level of the second field. As shown in FIG.18B in the third embodiment, in response to the change in gray level inthe third field, the driving circuit 10 supplies as a driving voltage anovershoot voltage to the liquid crystal panel 15 in the third field.More specifically, this overshoot voltage is a voltage overshooting (bythe overshoot amount OS in the figure) the gray-level voltagecorresponding to the input image signal S of the third field (thisgray-level voltage is applied in the four and the following fields).From the third field, the input image signal S does not have any changein gray level. Therefore, the driving circuit 10 supplies as a drivingvoltage the gray-level voltage corresponding to the input image signal Sto the liquid crystal panel 15 without overshooting the gray-levelvoltage.

[0246] As is apparent, the overshoot gray-level voltage (having its highband being enhanced) is supplied to the liquid crystal panel 15 in thirdfield, whereby the response characteristics are significantly improvedover the conventional LCD (dashed line in the figure) in which anon-overshoot voltage gray-level voltage is applied.

[0247] Note that an interlace-driven LCD in which a single fieldcorresponds to a single vertical period has been described in thepresent embodiment. However, the LCD according to the second aspect ofthe present invention is not limited to this, but can also be applied toa non-interlace driven LCD in which a single frame corresponds to asingle vertical period.

[0248] According to the present invention, an LCD having an improvedfall response speed is provided. In particular, by applying the presentinvention to a parallel-orientation liquid crystal layer, the responsetime can be reduced down to about 10 msec.

[0249] The LCD according to the present invention has a high responsespeed. Therefore, blurred image resulting from the residual imagephenomenon in the moving picture display is prevented from beingproduced, allowing for high-quality moving picture display.

[0250] According to the present invention, by setting the capacitanceratio of the storage capacitor Cs to the liquid crystal capacitor Clc(Cs/Clc) to 1 or more, the response speed (step responsecharacteristics) of the charging characteristics of the picture-elementcapacitor is improved. Accordingly, when at least the highest gray-levelvoltage is applied, the picture-element capacitor Cpix retains 90% ormore of the charging voltage over one vertical period. Therefore, an LCDwith improved response characteristics in a high band (a high gray-levelvoltage region) is provided. Moreover, for an intermediate gray levelhaving a low response speed, rapid response is implemented by overshootdriving.

[0251] By applying the present invention to a display device of a liquidcrystal mode having both wide viewing-angle characteristics and arelatively high response speed, an LCD having both wide viewing-anglecharacteristics and excellent moving picture display characteristics canbe implemented.

[0252] While the present invention has been described in a preferredembodiment, it will be apparent to those skilled in the art that thedisclosed invention may be modified in numerous ways and may assume manyembodiments other than that specifically set out and described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention that fall within the true spirit andscope of the invention.

What is claimed is:
 1. A liquid crystal display device, comprising: aliquid crystal panel including a liquid crystal layer and an electrodefor applying a voltage to the liquid crystal layer; and a drivingcircuit for supplying a driving voltage to the liquid crystal panel,wherein the liquid crystal panel exhibits, in its voltage-transmittancecharacteristics, an extreme transmittance at a voltage equal to or lowerthan a lowest gray-level voltage, and the driving circuit supplies tothe liquid crystal panel a predetermined driving voltage overshooting agray-level voltage corresponding to an input image signal of a currentvertical period, according to a combination of an input image signal ofan immediately preceding vertical period and the input image signal ofthe current vertical period.
 2. The liquid crystal display deviceaccording to claim 1 , wherein a difference in retardation of the liquidcrystal panel between a state where a voltage is not applied and a statewhere a highest gray-level voltage is applied is 300 nm or more.
 3. Theliquid crystal display device according to claim 1, wherein the liquidcrystal panel is a transmission-type liquid crystal panel, and theextreme transmittance provides a maximum transmittance.
 4. The liquidcrystal display device according to claim 1 , wherein a single verticalperiod of the input image signal corresponds to a single frame, at leasttwo fields of the driving voltage correspond to a single frame of theinput image signal, and the driving circuit supplies, at least in afirst field of the driving voltage, a driving voltage overshooting agray-level voltage corresponding to an input image signal of a currentfield to the liquid crystal panel.
 5. The liquid crystal display deviceaccording to claim 1 , wherein the liquid crystal layer is ahomogeneous-orientation liquid crystal layer.
 6. The liquid crystaldisplay device according to claim 1 , wherein the liquid crystal panelfurther includes a phase compensator, three principal refractive indicesna, nb and nc of an index ellipsoid of the phase compensator have arelation of na=nb>nc, and the phase compensator is arranged so as tocancel at least a part of retardation of the liquid crystal layer.
 7. Aliquid crystal display device, comprising: a liquid crystal panelincluding a plurality of picture-element capacitors arranged in amatrix, and thin film transistors respectively electrically connected tothe plurality of picture-element capacitors; and a driving circuit forsupplying a driving voltage to the liquid crystal panel, wherein theliquid crystal display device updates display every vertical period byrendering the plurality of picture-element capacitors in a charged statecorresponding to the input image signal, each of the plurality ofpicture-element capacitors includes a liquid crystal capacitor formedfrom a corresponding picture-element electrode, a counter electrode anda liquid crystal layer provided between the picture-element electrodeand the counter electrode, and a storage capacitor electricallyconnected in parallel with the liquid crystal capacitor, a capacitanceratio of the storage capacitor to the liquid crystal capacitor being 1or more, and the picture-element capacitor retains 90% or more of acharging voltage over a single vertical period, when at least a highestgray-level voltage is applied.
 8. The liquid crystal display deviceaccording to claim 7, wherein the driving circuit supplies to the liquidcrystal panel a predetermined driving voltage overshooting a gray-levelvoltage corresponding to an input image signal of a current verticalperiod, according to a combination of an input image signal of animmediately preceding vertical period and the input image signal of thecurrent vertical period.
 9. The liquid crystal display device accordingto claim 8 , wherein, for the input image signal of every gray level,the driving circuit supplies to the liquid crystal panel the drivingvoltage overshooting the gray-level voltage corresponding to the inputimage signal of the current vertical period.
 10. The liquid crystaldisplay device according to claim 7 , wherein the liquid crystal layerof the liquid crystal panel includes a nematic liquid crystal materialhaving a positive dielectric anisotropy, the liquid crystal layerincluded in each of the plurality of picture-element capacitors includesfirst and second regions having different orientation directions, andthe liquid crystal panel further includes a pair of polarizers arrangedso as to orthogonally cross each other with the liquid crystal layerinterposed therebetween, and a phase compensator for compensating for arefractive index anisotropy of the liquid crystal layer in a blackdisplay state.
 11. The liquid crystal display device according to claim7 , wherein the liquid crystal layer is a homogeneous-orientation liquidcrystal layer.
 12. The liquid crystal display device according to claim11 , wherein the liquid crystal panel further includes a phasecompensator, three principal refractive indices na, nb and nc of anindex ellipsoid of the phase compensator have a relation of na=nb>nc,and the phase compensator is arranged so as to cancel at least a part ofretardation of the liquid crystal layer.